I used to dual boot Windows XP (home) and whichever linux took my fancy at the time.
I found that by keeping it as basic as that, was a bit of a nuisance, because whenever I just installed something over the top of whatever linux I was using at the time, I always had to set up all my fav' customisations again. I didn't know how to get round that.
Then I tried gentoo (still my favourite distro, but I've gone back to mandrake/mandriva, because gentoo really requires more knowledge of how stuff is done manually than I have/have the time to learn).
The only way to install gentoo, was at least by using their default partitioning scheme (I did try to do it like mandrake/mandriva does by default i.e. everything in one partition, but instead of different partitions, the /boot, /home etc are directories). So I did that by modifying my partitions table.
I believe I'm correct in saying, that you can still only have 4 primary partitions. I think (might be wrong) that theres also a limit to the number of logical partitions that you can have, but it's one hell of a lot, compared to primary partitions.
So I had
/dev/hda1 as primary for the XP
/dev/hda2 as primary for the /boot (I used 1 gig of space, not that I needed that much room, but it was a nice round number and I had enough space).
/dev/hda3 as primary for the /swap (I.5 gigs, following the original linux wisdom of having a /swap of 2 x installed ram - I have 768 so 1.5 gig was nice and round and about twice the ram!)
/dev/hda4 was then set as extended into -> /dev/hda5 as logical for /root and /dev/hda6 also as logical for /home
One of the apparent bonuses of linux/partitions and the like, is that it would have probably been just as easy to have had /dev/hda2 set as extended so that I could have then had /hda5 as /boot, /hda6 as swap, /hda7 as /root and /hda8 as /home (all logical) and that would have left me with 2 potential primary partitions to do with what I wanted.
It's all about that rather annoying linux thing, known as choice. Which is great, if you know enough to know which are the best choices to make (or more specifically, which are the best choices to suit what you want to do).
For instance, you could just start with say, partitioning stuff up, so that you have 1 primary (erm, presuming that you don't want windows) and extending that so that you had, say, 5 logical partitions, onto which you put, /boot, /swap, /home and /root (mandrake for example), leaving one empty. Let the installer do it's thing (though you would have to have an idea about telling the installer what you wanted, and where), bung the bootloader onto the first section of the MBR (Major Boot Record).
Then, while you concentrate on learning that to your satisfaction, also install a different distro onto the 5th empty partition, but make that so that it boots with a floppy (a lot of the mainstream distros seem to offer that option). That way, you shouldn't have to worry too much about screwing up your bootloader i.e. if the second linux distro didn't detect the presence of the first one, you'd only be able to boot into the second until you'd worked out about how to repair the bootloader so that both distros boot.
I would suggest though, you should get familiar with one distro first, then if you do screw things up you have an idea of how to repair it. You should also have a look too see exactly what you need to have, filewise, to get it to boot e.g. kernel images, initrds', system maps etc etc plus whichever bootloader you use (I used to always use Lilo, but since my foray into gentoo, I've learned about and now prefer grub).
My only real bit of advice, is that you MAKE SURE that you have a seperate /home, then (in theory) you shouldn't loose all your data i.e. customisations, email settings/addresses etc, and if you did screw up, things should be recoverable.
It may also be worthwhile learning about /etc/fstab files, as you might want to be able to look into different /root file systems from different distros and stuff like that.
Sorry I can't be more "instructional", but as I mentioned, it's a good while back since I was meddling with stuff like that. I stopped, because I don't "do stress" and it was really pissing me off, that everytime I'd try to install extra distros, I'd screw my system up, because I didn't have much of a clue as to what I was doing (HA!, still don't really).
Hopefully, that will give you something to think about/look into.
p.s. Oh, and hopefully my post isn't too patronising/condescending, and that you don't turn out to be an experienced Linux/Unix sys admin with a Phd. in IT just out for a bit of a Troll (which would be just my luck! especially if my suggestions/ideas are total rubbish that only really make sense to me!
Good luck with you linux adventures.