The 64-bit version shouldn't be any faster than the 32-bit version, but it would be great if someone could do a side-by-side comparison of the two linux versions and check. SHA-256 is a 32-bit algorithm and nothing in BitcoinMiner uses 64-bit at all.
But look here:
32-bit Linux version on Ubuntu 10.044 cores: 2500 khash/s
3 cores: 1900 khash/s
2 cores: 1260 khash/s
1 core: 630 khash/s
64-bit Linux version on Ubuntu 10.04 (new measure)4 cores: 2880 khash/s
3 cores: 2150 khash/s
2 cores: 1450 khash/s
1 core: 720 khash/s
(Though I never created one coin, yet - but I don't let bitcoin run on 4 cores the whole day, not even on 1 core...)
@Joozero - The Intel i7 860 is @ 2.8 Ghz, isn't it? My Phenom II is @ 3 Ghz, that's another important fact I think. I will try bitcoin on Windows 7 at the same machine later.
/edit
32-bit Win version on Windows 7 64-bit4 cores: 2310 khash/s
3 cores: 1740 khash/s
2 cores: 1160 khash/s
1 core: 580 khash/s