What a good question to answer. I suppose it's really a question of "how big is your budget".
The following suggestions are extracts from a "n00b" tutorial in Linuxformat issue 43 (don't know if they've got it on their archives yet).
"and anything above 500 mhz will be perfectly adequate"
"You can run a PC on a tiny amount of hard disk space, but if you want it install KDE, GNOME and a decent selection of ap0lications you really need something in excess of 3GB of free space. The latest Redhat, SuSE and Mandrake distro's claim a minimum of 2GB for a full graphical install, but if you scrimp here, you may spend a lot of time looking at worring messages saying you're about to run out of disk space. If you have a faily large disk and you are dual-booting with windows, 3-5GB lor a linux partitions will usually be enough for the system and basic applcations"
Don't forget, that if you are buying new, then you won't see hard disc's much under 40 gig's in size - small ones tend to be rare and expensive. I would probably go for maybe 2 80gig drives if I was starting from scratch, and they would be SCSI disc's as opposed to IDE type disc's - but SCSI is more expensive.
"As with hard disk space, you can run a Linux PC on a tiny amount of memory - but why would you want to when RAM is so ludicrously cheap"
So in other words, go for as much and as fast as you can afford - if you're buying a new PC it's worth remembering that it's often even cheaper to buy it at the point of manufacture/build, than it is to just buy extra later on (and that isn't too much either).
"A bootable CDROM is almost essential" and "DVD drives and CDRW (Rewritable) drives are always handy to have, and accessing them shouldn't pose a problem for most Linux distro's"
It depends on kind of net access/connection you have or can have either way from my point of view, I would go for an NIC ethernet card, and external modem (though you should look
for a run down of compatibility)
GRAPHICS AND SOUND
"Whether your PC is equipped with an integrated graphics and sound system, such as intel's ubiquitious i810 chipset, or a high-end AGP graphics card and pro-quality sound card, you should not encounter any problems during installation"
It's just worth bearing in mind, that with very mega new graphics and sound card's, you may experience driver problems, though these are usually sorted out relatively quickly.
"Keyboards, mice, monitors and printers are all well supported. In fact when it comes to printers, many older models have better drivers for Linux than for Windows, thanks to the efforts of Open source developers trying to get the most of their existing hardware"
Well, that's it for extracts from the mag (with my comments - mine are the bits without quotation marks).
As I said, it's really about how much you've got too spend. I tend to favour the as much,and as fast as possible approach.
If the budget is good, then have a look here
at the "workstation's" they're expensive, but look like a serious piece's of kit - and it gives you an idea of what "high-end" really means, but don't forget, the extract's that I've included indicate that a couple of hundred A$ worth from a computer fair should also suffice.