I am considering the same question. I've read several comments from people who have used the 64 bit OS and say they are having good luck with it. Here are some tidbits of info:
1) Check to see if your peripheral devices have had 64 bit drivers created for them. If not, you cannot use them. For example my printer/scanner/copier by HP is reported to have the 64 bit driver available even though the actual model is no longer being sold. I found the info on the HP website.
2) 32 bit programs will run on XP64
3) 16 bit programs will not run on XP64
Since my next computer build is being done for my own personal learning experience and I will have my old computer available for when things go wrong, I am probably going to go with the 64 bit OS.
I just built a new AMD X2 4200+ system and used win XP 64-bit. It worked allright, looked and felt exactly like the 32 bit version. Fortunatly most of my divices worked fine, but i did have to get the 64 bit driver for my Eposn printer. I am finding that there are alot of programs that wont work though- particularly anti-virus and disk utilities. and the two "Program Files" folders and 2 versions of internet explorer are very confusing. I am probably going to revert back to the 32 bit version later tonight and just wait for Vista 64 to come out.
I am really wondering though that performance losses i will experience by reverting back to a 32 bit OS on a 64 bit dual core machine.
If you feel the need to ask you should probably go with the 32Bit version. I'm using a newly built X2 system and went for the XP 64 option, I haven't missed the normal version of XP one bit but then I did check all the drivers existed before I installed and burned them all to CD. The only program I'va had problems with was the new Splinter Cell which refuses to run due to using an old copy protection system which doesn't work on 64 bit.
As for Antivirus I use NOD32 which has a native x64 version, Norton Corperate which I tried first was crap and buggy, that aside it's slightly faster than XP32 in normal use but not with 32 Bit apps which run in WOW64 emulation at the same speed as they do on 32Bit windows.
The biggest problem people have with x64 windows is driver availability. The major manufacturers provide 64-bit drivers now, most of them now getting out of beta. I personally run the 64 bit of windows on my desktop, I would run it on my notebook if it was 64-bit capatible, and given my hardware is from major manufaturer's there is a few versions of 64-bit drivers to choose from. The only beta drivers I am running are for the Creative X-Fi and the nForce 3 250 chipset, both of which are completely stable.
The only program/game I have not been able to get to work is Splinter Cell Chaos Theory but this is a known issue with Ubisoft but they are not doing anything for it. Eh, oh well.
I have read people experience performance losses with 32-bit programs in x64 windows, I have not experienced any losses.