This post can basically be summed up by this VMware KB article; however, arriving at this bit of knowledge took some trial and error, to say the least, and I still don't know the whole story.  Maybe someone out there can fill in the gaps.

Until quite recently, I've been running a home-built AMD Athlon 64 3500 (2 GB RAM; SATA; MSI mobo) based system.  Circa last year, that was a sweet system for the money, especially since the proc was fast and cheap.  I've even been running 64-bit XP on it, and the only major challenge has been the non-availability of a driver for my Canon multi-function printer.

I've lately had the need to do some 64-bit testing, though.  I typically use a VM for this, so as not to toast the host system, and the beauty of it all is that the latest VMware workstation product supports 64-bit.  I installed the 5.5 demo on my 64-bit XP system and got various 32-bit guest systems running in no time. 

When I finally tried to install a 64-bit guest, though, VMware rejected it, saying that my host wouldn't wouldn't support them (I tried 64-bit SUSE 10, as well as XP 64).  I thought - bull sh*t; I've got a 64-bit proc running a 64-bit OS - what's the problem! 

Well, come to find the VMware KB post linked above.  I also found a tool called CPU-Z at http://www.cpuid.com, ran it on my system, and discovered (along with lots of other interesting details) that my Athlon uses 130nm CMOS technology.  Therefore - not capable, from the perspective of VMware, of running a 64-bit guest.  Here's the $64 question - does anyone know why, technically, this is?  I searched briefly and couldn't find a good explanation.

Anyway, I just bought a Dell Precision 670 (Xeon/SCSI) :)  And - that proc is 90nm, not that it matters ...