Do you have any problems with the XP home that came with your computer? If so, go for the xp64, I'm a beta tester for microsoft, and I'd say that xp64, is very good. The benchmarks are withen a percent or two of xp32. That may change as the drivers for xp64 mature. I have no problems with xp64 sp2(beta) other then no modem, big deal! My machine runs great. Guild Wars plays great, HL2 played just as good on 64 as it did on 32. So really it is up to you. The new "VISTA" will feature a new 3d graphic interface, I'm sure that will go through alot of changes before the final is realesed. Microsoft has been known to not support old "Legacy" hardware with their new OS realese, so it would not supprise me if there is no support for out older AGP 9600 Mobility, it does not support Pixel Shader 3.0, so if that is a requirement for the OS to function we will be out of luck anyway.
I guess what I'm saying is this, if it is not broke, don't fix it. If you want to play with the 64 bit OS give xp64 a try you might like it. I think it works great, I also think XP32 Home is just fine for almost everyone. It is fast, stable, powerful, and did I mention STABLE.
Keep us posted on what you do.
(Adrian Brock in Guild Wars)