Anybody with Vista has little reason to upgrade. Exceptions...
Windows 7 is going to be tweaked for snappier performance. The lower the specs of your computer, the more noticeable this will be.
There should be no gaming performance difference on most computers, and none on any high-end system. Systems with low RAM in particular will see better performance under Windows 7 for general PC usage and MAY see a slightly better performance playing a game if RAM is low, but if it's a RAM problem you'd be far better off buying more RAM than buying Windows 7.
4GB is optimal. Any more is generally a waste of money, and any less will start to degrade performance; I've noticed a difference from 2GB to 4GB for basic desktop use. At 1GB Windows 7 will be much snappier for general use than Vista.
The hard drive is also important. For basic use, such as multitasking e-mail and internet the hard drive and sufficient RAM are the most important factors. Laptops often feel sluggish due to low hard drive speeds, though enough RAM can often offset this.
Windows 7 uses the same driver base as Vista basically.
So my answer is if you don't have enough RAM, buy more (4GB if possible). If your hard drive's too slow, buy a faster one (or both).
XP is still a great operating system. However, I think a lot of people would benefit from an upgrade to Windows 7, 64bit if supported. There are stability reasons as well as performance ones. I don't understand all the reasons, but XP tends to get more sluggish over time where Vista does not. Vista Aero (and Windows 7) look much nicer on my monitor due to the new graphics support (much, much nicer). Do check driver support, especially for peripherals but don't expect to see the drivers quite yet for everything. Support should be really good overall.
Vista to Windows 7?
Not really worth it. XP yes for many. It does depend on how picky you are and remember my point about the hardware. Better money can often be spent on the RAM.