Well if you want to get Vista , that system is capable
From my experience i dont use VIsta cos i didnt feel any benefit from using it , in fact at times i found it quite distracting and annoying , and the new UI isnt exactly anything special to me , not worth £150 anyway
But on the other hand it has some realy good features such as an easy to use networking center and its supposedly more secure although i never really noticed that
All in all if you like the look of vista , look at some screenies then run it on your machine , if you dont like it then get rid of it and get XP .
Obviously this is illegal but what i would do is get hold of an illegal copy of vista and run it and see , then if you like it buy it and if not get XP
I dunni if vista actually has a trail version , if so try that instead of being illegal
My opinion on this is try both chose what you like . I doubt there will be any performance differences between machines
I did that and now i know which one is for me