That is slightly better than my old system that was able to 5 box, but when entering big towns (stormwind or dalaran) it would lag alot and break follow, had to move them individually. But outside of town I was able to 5 box OK.

Your processor is a step up and video card is much better than my old one, so if you go by that you should guess you'll be able to 5 box ok. WoW, but not Rift.

Anyhow I think the current version of vista is 6.1 (but not positive) you should try windows update just in case.