Currently Intel's line of 64-bit chips that support Intel Virtualization Technology (VT) (Vanderpool/Silverdale) are the best choice for use with virtualization software. This includes their laptop and desktop chips, the Core 2 series and their Xeon serious server chips. AMD has their own chip-virtualization technology called AMD-V (Pacifica), but it is currently only offered (according to the AMD Website) in their Opteron series workstation/server chips.
Even though AMD offers chips with similar functionality, I recommend Intel because although both VT and V are relatively new, Intel-VT has been out longer and is likely to have more support from virtualization software vendors.
I do not get into many 32-bit vs. 64-bit debates because I usually do not think the extra width makes a lick of difference. However, it is important when designing a new virtualization infrastructure to go with 64-bit chips, because a 64-bit enabled host server is required to host 64-bit Virtual Machines (VMs). Since VMs are often used as test beds for new software or developing applications, you will want to be able to easily deploy a 64-bit version of an OS so you can test a 3rd party application, or your own application, on it.
This was first published in September 2006