well first of all, the raw power increase is going to be worthwhile. I'd suggest a nice 2xCPU server from that pcsforeveryone.com site or whatever it was called (they had some good deals) and customise for one CPU that's dual core - you can add another CPU later. The dual cores are sorta like getting two CPUs in many ways.
Also, ALL AMD64 chips are FULLY backwards compat with ALL 32 bit operations. They can natively run 32 bit stuff with NO speed penalities - though it is nice to get the extra 32 bits when you find a 64bit program. I gotta run now, seeya later.