I used xp64 for a few weeks before. For me, it worked just like 32bit except without the support of as many programs. I had 2gb ram at the time, and I'm sure if I added up to 4gb (my current config) it would have performed better, simply because the architecture of the OS would support and utilize 4gb compared to xp32. I had no problems with it--same installation, same functioning (as far as I could tell). What I didn't like was the lack of supported programs. Many of the programs I use are basic Notepad-created programs, many of which apparently are not supported by xp64. Thus, I got rid of it and went out and got xp32 pro.
But, of course, if you are comfortable without all the support of a wide variety of programs, by all means upgrade. you seem to know which programs you want that do support the architecture. But make sure you get an AV and spyware remover that support xp64. SpySweeper does not, nor does Norton 360 (I attempted to install these and neither worked. I'm sure these are made as a separate program just for xp64, though).
hope this helps!