Quite frankly, Vista doesn't give you much of anything that you can't have in XP with a couple free downloads from cNet or PC World or whatever. IMHO there isn't any driving reason to upgrade, aside from curiosity/hype.
(Which I dutifully fell for ) Especially since you don't use your computer for anything you'd be even vaguely limited on in XP. Now, Vista isn't quite the massive drain on hardware which some would have everyone believe, but it *is* more for your comp to much on. Not to mention the potential maze of driver headaches caused by vendors not updating their software.
I know this may potentially come across as a dig against the OS (it isn't), or some juvenile backhanded way to put down a "lesser" system - PLEASE do not take it as that. I have Vista, have only had to deal with minor hiccups, and generally like it. But the simple truth of the matter is that your older system will be happier and will run better with XP.
If you're like me and will *have* to play with it anyhow because that's the only way to get the bug out of your ear, I'd recommend a dual boot - Buy yourself a second HDD, set it up for Vista, and leave your current OS and information intact and, well... unmolested.
At less than $100 for a decent HDD, a few minutes to add it to your comp, and the install process you would have gone through anyhow, call it cheap insurance.
MSFT do have an Upgrade Advisor application you can download from the Vista home page. Definitely do that and run it. It'll read your system's hardware *and* software, then return a list of issues which you'll be able to go through and judge for yourself whether or not the side~grade is worth it.
Scott