Yeah, it is a bit of a RAM hog, but RAM is so cheap right now that if you're buying a new computer for games you shouldn't be putting anything less than 4 GB in it (dual channel). That said, XP is only going to allow you to use 3 GB of that, so the other GB that you paid for is pretty much just sitting there useless (and because of the pricing, it's pretty much free when you consider the price of a 4 GB ram kit over that of a 2 GB kit and a 1 GB stick). In my mind that makes up for it. Beyond that, what Scott said; it's a RAM hog, but in good, useful ways.
The things I don't like about Vista are more in line with what Que is talking about; it asks for permission for too many things (you can probably turn this off), and other little tiny annoying things like that. Beyond that, a lot of the standard windows functions have been moved around and/or renamed making it a bitch when you try to do some things such as setting up a home or print network. For the casual user though, I don't know how much of a difference it would make and it may even be more intuitive.
There are a few things I really like about it though; aero glass is nice on the eyes, the task bar preview feature is something I've found to be very usefull when I have like 6 different office windows open, and little stuff like that.
In the end it's kind of a toss up, so I'd just do whatever Scott says....he's really really smart.