I agree with dave...go with a 64 bit, it's no contest. The question you have to ask now is what flavor. Personally, I've got the 64 bit version of all 3 operating systems installed on various pcs. If it were my pc, and I only had one, I'd dual boot with XP 64 and win 7 64. I'm running the 64 bit version of Vista but I don't like it. Too much overhead even after I've scalped it. XP works without all of the processes that it takes for a useless transparent clock! And Win 7 looks good so far, but you also need reliability, thats why you should think about XP.
[/my opinion]