Search for running ESX under workstation or check out the Xtravirt whitepapers on the topic. Basically, ESX3.5 requires large page support (gets huge speed improvement). To run ESX 3.5 under Workstation 6.5 you need a CPU that supports large pages. So early 64-bit CPUs - no go, more recent CPUs (ie one- year old or so) or quad-cores - generally OK
So what you are runing into is NOT an issue of 64-bit. IT has to due to what ESX 3.5 is looking for in terms of CPU support.