I've got 32bit Centos 5 running on a 64 bit Centos 5 host. VirtualBox initially failed as it couldn't compile it's kernel so I had to do some Googling and downloading as Centos 5 apparently doesn't have the source files by default and I'm not sure if they're available from them. I wish I could remember where I got the solution from... Rats!
Note that I couldn't get 64 bit to run as a Guest as I "didn't have a suitable CPU" I'm using an HP ML 370 G4 with dual 3.6Ghz Xeons.
I'll get back if I can remember what I did. Meanwhile...