go for vista. the only reason to stick to XP would be if your hardware aint upto the mark, yours clearly is... oh! another reason would be if you are one of those who like to check how much free RAM do they have at any moment. vista caches a lot of free RAM. people need to understand that RAM is a resource. not using it is a very stupid thing to do... there is no such thing as good free RAM... it is jsut wasted RAM. if the OS consumes it to be faster, its a good thing. caching RAM means just that, use it when its free and release it when other apps require it. people who have worked on exchange would know how good and effective RAM caching is...
however in the end.. the choice of OS is just not about the speed and free RAM.. it should not be... vista is better overall on every front when compared to XP...
another thing... do not go in for x64 if you do not have any dedicated use for it... most apps now are still 32 bit and they run in a separate subsystem on x64 OS.. its like a 32 bit environment for the app specifically.. (these show up with a "*" with their names in the task manager).. so running a 32 bit app on a 64 bit machine involves overhead... before someone gets upset.. i use vista x64 SP1 but i do so coz i run 64 bit guest OSes on vmware...
the only other reason to go x64 is if you are planning to upgrade to 4 gigs of RAM or more...