linuxhardware.org and anandtech.com have been benchmarking 32 vs 64 and AMD 64 vs Intel 64 specifically on Linux for a couple of months now. See http://www.linuxhardware.org/search....ction=features http://anandtech.com/it/ and http://anandtech.com/cpuchipsets/

If I summarize the tests then I will probably make a mistake. I will say that the tests have shown that the AMD 64 platform (Athlon 64 & Opteron) is better than the existing 32 bit platform (both AMD Athlon & Intel Pentium 4) and in most it is better than Intel's 64 platform (Intel P4 64 bit extended & Intel Xeon 64 bit extended).