The answer for that really depends upon 1) the size of the environment 2) resources available for the vCenter server.
I don't believe I've ever seen a real recommendation from VMware, I generally start looking at a separate SRM server with 150 or more virtual machine OR the chance of growth in the foreseeable future. It can't hurt to just install as a separate server, especially if vCenter is a 32-bit server.
I would always recommend a separate windows instance for SRM...
As for 64/32-bit. I'm making the move over to W2K8-64-bit in all my VMware infrastructure where I can... I think we will be facing a 64-bit on version of vCenter in future - it kinda leaked out the rel notes for the last release of vCenter 4.0 U1
Author of the SRM Book:http://www.rtfm-ed.co.uk/2010/03/22/new-administrating-vmware-site-recovery-manager-4-0/
Free PDF or at-cost Hard Copy