Most important thing you have to think about upgrading to Vista:
1) Is your computer powerful enough for Vista?
2) Is your most used software compatible with Vista?
3) Do you already need Vista?
Vista has very high requirements for computers hardware and on the same hardware, Windows XP definitely will run faster, but there are some features which XP does not support.
One of those features is large amount of RAM. As Windows XP is 32bit application, it supports only 4GB of RAM. If you want to use more, You need Windows XP 64bit, or already Vista.
Most of software developers still have not prepared their software for Vista. If you have some expensive software, first check is there are updates for full compatibility with Vista, or even new releases. For example Photoshop CS2 will run on Vista, but Photoshop CS3 is designed for Vista.