TBPH, the biggest problem people have with Vista over XP (ie the load times, freezing, file xfer speeds) exists not with the OS itsself, but with the computer it's being run on.
XP takes 64mb of your total ram to operate. By today's standards, that's very lightweight. However, it wasn't always the case, as when it was released most machines came with 128-256mb of ram as standard.
Now when Vista released, many people and businesses upgraded their OS only to find the aforementioned issues. Vista is a memory hog, and therefore does not do well on machines over a couple years old. For myself, I have 4GB of ram on my machine, and a video card that uses it's own dedicated memory, therefore I rarely experience alot of the common complaints with vista. However, if memory is your issue you either need to downgrade back to XP or invest in a better machine.
With Windows 7 in beta, it's hard to tell whether or not it will be a huge upgrade from having Vista. As it stands, Windows 7 is quicker on load times for general applications than either XP or Vista. However, with high end gaming, Vista (64bit) still comes out on top, giving slightly better graphical performance than Windows 7, and also better frame rates, on an average of 4 FPS (frames per second) more. These figures could change once we have access to Windows 7 specific graphic drivers.
In conclusion, XP will suit those on older machines, or those who use the computer for word processing and internet browsing. At the moment, neither XP nor the new Windows 7 can beat Vista for gaming, as Vista boasts hardier 3D rendering capabilities, especially when it comes to games that implement Direct X 10. But it is recommended to have at least 4GB of ram for vista 64bit (and if youre in it for DX10 wielding games, chances are you have 4GB or more). And as far as sheer speed is concerned, Vista 64 bit still reigns, although as mentioned it could change once we have the proper drivers for the Windows 7 OS.
