I used to be a Staples Easy Tech Guy, I can tell you XP is much smaller, and there are those "Ways to maximize XP" books for about $20 that really make the thing fly, especially with 2GB of RAM.
If you really want an OS you can shake a stick at, try Linux. It is light as a feather and can really maximize applications, you just have to have a bit of know-how to start.
There are no real advantages to Vista unless you are looking for a multimedia PC, which I dont know why anyone would.