Actually, no, 64 bit processes are not double the speed of 32 bit processes. 64 bit refers specifically to the number of registers the CPU can use to hold memory. 64 bit processors can basically address more memory than 32 bit processors, thus why you see such a large jump to 4GB. A quad socket Opteron board would be able to support 32GB I believe.
The 2800+ would be the faster solution due to the better memory bandwidth over the XP.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w