If we can't talk you out of it
, I've seen a couple of tutorial Web site features that might be useful to you:How do I... Install Windows Vista in a dual-boot configuration along with Windows XP?http://articles.techrepublic.com.com/5100-10878_11-6157570.html
In order to install Windows Vista in a dual-boot configuration along with Windows XP, you need to have either a second partition on your existing hard disk or a second hard disk in your system. To give yourself enough room to experiment, you should have at least 20 GB and preferably 40 GB of space available on either the second partition or on the second hard disk.How to dual-boot Vista with XP (with XP installed first) - step-by-step guide with screenshots
[this one doesn't speak to drive space suggestions though]http://apcmag.com/how_to_dualboot_vista_with_xp__stepbystep_guide_with_screenshots.htm
I've toyed with the dual booting idea due to some Vista 64-bit incompatibilities with Palm PDA desktop synch software and Rhapsody's music store software.
Honestly, if you're happy with XP, I don't really see any reason to risk messing up your PC to "play around with it." Vista doesn't do anything for games aside from the very spotty DX10 support, and it's certainly not going to make any games run faster (it'll just increase your RAM requirements, if anything). I have no doubt my fairly new system (Quad Core 6700, 4GB RAM) would run games faster in XP, but I just have been hesitant to mess with dual boot OSs or partitioning.
Other than spiffier menus, a Start Menu where it seems actually harder to find programs, and a little gaming folder that all your games' get tossed into, there really isn't much to see in Vista. I've been using it a couple of months, if you have any questions about it, I'm happy to answer.
You might run by a Best Buy or Circuit City and play with Vista a bit there (that's what I did here and there).
That's my 2 cents anyway.
XP-wise, it seems kinda neat that it's finding a second life on all these endless Eee PC wannabes (mini-notebook computers), nearly all of which are coming with XP SP3 pre-installed now. So I dunno, maybe at some point Microsoft will market XP as a "mobile OS" due to its relatively modest system requirements.