As Microsoft has extended the support for XP probably there is no need to update to Vista, unless you buy a new PC and you cannnot choose your OS.
Vista uses more resources, as I updated with a new PC, a core2 duo, more RAM, more everything,... of course I did not notice a slowdow, the contrary. But it is true that needs more memory, I use Vista x64 (for some reason) and I think that needs even more. 40% of my 2GB are used after startup, and that after cleaning some services I do not use
I have been using Vista x64 for nearly a year without problems, once used the recommended tweaks in that site, UAC is really annoying and must be desactivated, just IMHO
Restore points use resources, but if you use your computer to work, probably it is not a good idea to get rid of it. As I use my home computer for fun I will not lose anything important, so I have desactivated
There are tweaks for XP too in that site
In my old PC with XP I had Page File set to 0. This option is not recommended by microsoft. But if you have enough memory and you do not use a lot of programs at the same time you can use it safely. At least I did it for several years without problems. RAM is faster than HD, so there is a noticeable speedup, that is the reason to set it to 0