XP 64? use or not

Building some computers for my friend and it just hit us we might need 64.

specs are intel core2 quad, p5e asus, 8600gt, with 4(1x4) gigs of ram.

these are for rendering with maya mainly and ive never had a need for 64 with my own computers. should i download, i mean buy, 64 or stick with oldschool xp

Also, AMD cleaned up the instruction set a bit for 64-bit mode. There are twice as many integer and FP registers (x86 is register starved for lots of applications). And the addressing modes were rationalised, either some were added or some that were never used were removed, I can't remember.

The end effect of this is that amd64 is faster than x86. This is different to most CPUs that were designed properly from the start (e.g. PowerPC) where more memory fetches associated with addresses twice as long actually slow things down unless you need >4Gb of memory.