I know it'd likely be more cost
effective to buy a new system with
AMD-V or Intel VT-x
AMD is cost effective. Intel, not so much. That's not to say Intel's processors are worse, but in terms of costs they're more expensive than AMD.
.. but to prove it, I need to know if
there are any 939 socket chips that
feature AMD-V and if so what they are.
Socket 939 or 940? Well, I highly recommend Socket F (barcelona/shanghai/istanbul all support AMD-V and are excellent for virtualization), but if it's 939 you need, check out this chart on wikipedia.
From what I can gather, the 939 opterons are all 1xx (1 CPU) per board and none of them list AMD-V compatibility. I can't believe I've never wondered if you can run virtualization w/o the extensions for the CPU. My guess is that the latest virtualization products need the extensions so it's a requirement for you.
It sounds like you have a 939, but you want to upgrade if you need to. If I can sway your decision, I highly recommend AMD over Intel, especially if this is for home use (I can't quite tell if your post is for business or home). AMD is usually more cost effective, has better upgradability (IMO - less socket changes, consistent memory requirements, etc.), and the energy-efficient Opterons are worth every penny.
AMD uses DDR2 (registered ECC) for DIMMs so the memory is bit pricer than some Intel solutions, but for virtualization servers, don't skimp on memory. They also have a fairly good array of CPUs for specific socket solutions such as the 1000 series (1 CPU/board), 2000 series (2 CPUs/board), 8000 series (4 CPUs/board). I can't tell what you're trying to virtualiztaion wise but AMDs product line (to me) is more cost effective and appropriate for what sounds like your first venture into the world of Xen.
Damn, I sound like an AMD rep! I'm not, but I admit I am a fan of the Opteron line these days.