Registered: Oct 2003
Location: Northeastern Michigan, where Carhartt is a Designer Label
Distribution: Slackware 32- & 64-bit Stable
If you're really sold on having an iMac, can't help you too much (personally, I kind of think that they're too proprietary and too expensive; just my opinion).
You may want to look at Dell -- I've had great experience with both Dell desk- and lap top machines on which I run Linux only. As it happens, I only use Slackware although I have tried other distributions (including Suse and Ubuntu). I prefer Slackware for a number of reasons, primarily that it is un-fooled-around-with (Slackware doesn't do a lot of thing for you -- or, more importantly, to you -- that you may or may not like done). In my experience Slackware is solid as a rock and I don't have any trouble with it.
So, back to choosing a box.
You may want to consider a 64-bit machine (rather than 32-bit) with a multi-core processor (two or more cores). Think about an AMD processor (uses less power and runs cooler than Intel) but Intel is fine, too. While you're at it, choose a processor that supports virtualization and, if you do choose a 64-bit machine, order it with at least 4G of RAM and think about 8G (or more) of RAM -- you're going to have this thing for a while and, given the lower prices for RAM nowadays, go bigger. Understand that a 32-bit box will not directly support more than 4G RAM (it will, but you have to twiddle it and you may not want to get into that).
You're most likely going to get a SATA disk drive by default and it'll be 250G or so (maybe larger). That's fine, 250G is a heck of lot of disk and you'll have a hard time filling it up. Might, though, be a good idea to get two drives and RAID them (RAID is the acronym for Redundant Array of Independent Disks; two or more disk drives where what is written to drive 1 is redundantly written to drive 2; drive 1 goes blooey, your stuff is available on drive 2 and you just keep going). Dells support multiple SATA drives (they bays are there, the wires and connectors are there, life is good).
Typical Dell desk tops have a large number of USB connectors -- these are good (one newer lap top I have has three sockets, not enough sometimes). A rule of thumb is "off you can take, on you can't put" so, the more the merrier.
You'll want a DVD burner. They'll read and burn both CD-ROM and DVD. This is a Good Thing: DVD hold a whole lot more stuff than CD-ROM (and they've gotten less expensive). I have no experience with Blu-ray Disc in computers (but they hold a heckuva lot more than DVD!); I know they're expensive (but getting more reasonable) and, maybe, there's such a thing as a Blu-ray drive that'll read and burn CD-ROM, DVD and Blu-ray that doesn't cost an arm, leg and three toes from the other foot. The technology is, pretty much, settled -- up to you.
Something I would consider for long-term storage of important data is a tape cartridge drive -- this is not a must-have, but it's one to think about for the long haul. Flash-drive, CD-ROM, DVD and, who knows, Blu-ray do not have an infinite shelf life (nothing does); they're not reliable for archival use -- even the so-called archival optical disk media can be expected to be useless in as little as ten years. If you've got stuff you really, really value, your two best bets for archival storage are hard-copy on acid-free paper and magnetic tape; and, yup, both of those have a limited useful life, too. Just something to think about -- I have 9-track tapes that are over 20 years old and I can still read 'em without error; I've got CD-ROM that are 5 years old and they're hanging from wires to scare the squirrels away from my bird feeder (can't read 'em, they're scrap).
Unless you have a crying need for super-duper graphics, I'd be happy with the stock, usually Intel, graphics controller. Either Nvidia or AMD graphics controllers are fine but the drivers are proprietary and even though the open-source drivers are getting better, they're not really 100% and you may -- may -- experience difficulties. I'm picky about sharp images and text and, so far, I'm OK with the stock Intel controller -- your mileage may, however, vary. Give some thought to the display you're going to be using (like the resolution of the display) and weigh the pros and cons from there.
Now, the reason I was going on about virtualizatoin up there is that you'll most likely get Win7 with whatever server you choose. Comes pre-installed, you pay the Microsoft tax, and that's that. However, it also comes with a CD-ROM or DVD so you can reinstall it if necessary -- that's where virtualization comes in. You can make your machine Linux only (you just repartition the disk(s) when you install Linux). Then, if you really enjoy dealing with Windows, you can install it in a virtual machine from the media you receive with the system. And, if you're getting a 64-bit box, that's why you need to pay attention to the processor having the capability of virtualization -- Intel does this nasty little trick where they turn off that capability in their lower-priced processors (even though they're multi-core). You won't be able to install 64-bit Win7 in a virtual machine unless you processor will permit that -- not a big deal, just look at the specifications for the machine and do a little Google work to find out.
It's pretty typical that installing Linux on something like the above is insert the distribution disk, boot it, configure your disk drive, setting up partitions and the like (a little more complicated if you RAID), then doing the installation. It's really just about that easy. You do need to know the address of your router or DSL or whatever internet connection you have but that's pretty much it for initial start up. You do need to read the information provided with the distribution you choose -- and, of course, there is a lot of help and advice available here at LQ in the Linus Forums section for each distribution...
I'm recommending Dell, there are, of course, other manufacturers. Do some searching, look at their web sites, think about what your needs are (and, hey, your budget!). Keep in mind that computers nowadays are pretty much commodities, they're all similar and you want to choose what gives you the bang for the buck. Avoid the package that gives you a free printer (they're cheap, many don't work with Linux). You probably want the mid- to upper line; i.e., the $200 one isn't going to be a good choice.
Be prepared for a learning curve and spend some time reading introductions to Linux before you make the leap -- that may save you some frustration. Some distributions, say, Ubuntu, come up running and you can clickty-click your way to nirvana. Others, notably Slackware, also come up running but give you the opportunity to actually learn about what's under the hood and how you can tweak it to fit your needs. I kind of like that.
Anyway, hope this helps some.