As RFactor, GTR Evo and other ISI / Simbin sims are using the same game engine i think it should run on 64 bit very good. I use 64 and 32 bit and i own almost all sims, they all run ok here.
Whats more important is drivers, read on Guru3d forums first when there is a new videodriver, if the comments are good then change to another driver.
Look also to you other hardware, if you own a pci tv card you can get some driver problems, so see if the rest of your hardware supports 64-bit.
So if you don't see other driver problems ahead, go ahead with 64 bit, but use 4GB as a minimum of ram.