Well from the sounds of it, 64 bit is supposed to be faster. I cant see a big diffrence. Thats about it. Things are supposed to be much more 64bit oriented once vista is out. So 64 bit might be the way to go now.
Well, i have the Athon 64 4200 which is 64-bit and i installed XP Pro 64-bit edition on it, and didn't notice any speed differences. THis is probably because most software is only now starting to release stuff that utilizes the 64-bits, whereas most of the stuff is 32-bit, which makes the 64 useless really. But as the previous poster said, now that Vista is coming out, i guess things will use 64-bit more, so in coming months, yes there will be a good difference.
64 bit can handle larger amounts of data. For example, windows XP 32 bit can only handle 2GB of RAM, it can handle more but it will only address 2GB I believe. It's possible to have more on a 32 bit system but there are stability issues from what I've read of people using 3 or 4GB or more. 64 bit XP on the other hand can handle up to 128GB of RAM and 16TB storage. I'm not entirely sure on the whole 64 bit thing either but that's what I can gather from it.