Almost all desktop processors being sold now are 64-bit, and you can run 32-bit operating systems and programs just fine on a 64-bit (AMD64/EM64T) PC, so you won't lose out on anything by choosing a 64-bit processor.Having a new system and xp x64 (wich compared to other MS OS is a bargain at the moment (and based on the Server Edition, not XP)) I am feeling pretty geeky at the moment - lotsa new experiences, new insight into windows in general, as it was in old times driver troubles (availability (better to say the non-availability) for medium (not included in windows) peripherie like most canon printers, and there are not so many AVs available for x64 at the moment and no app.based firewall Ghostwall is Port-Ruled, ZA is beta, as is tiny (AFAIK)). But in general its fun, really - Even if I disliked the feel of XP Home on one of my machines (the OS was included - i would have prefered 2k) - this one has the right feel (alike to Win95b, Win98SE and Win2k - None of those had ever really disapointed me (instead to Win95, Win98, luckily I avoided ME). Ok, the wow64 emulation layer for 32 bit feels somewhat like having one operating system the 64bit part wich at the moment only few apps./codecs/etc. really made use of and the 32bit layer on wich most is used at the moment (but times will change).
[Example AV-codecs and apps for them on the 64bit side could just access itself, as those installed as 32bit can only access the 32bit installs (example: VDub x64 can only access the 64bit VideoCodecs, VDub x32 can only access the 32bit ones)...
So if you are not the adventerous type of person or knowing your system it maybe is better to stay 32bit (on 64bit machine) at the moment. But in my eyes, it was the right decision.