memory - Lots of DDR2 and older CPUs or small amount of DDR3 and a newer CPU?

Building a new desktop PC, for some gaming and lots of Virtualization, I have come across a nice old machine I can use. It has two CPU slots populated by dual core AMD Opterons, and 16Gb of DDR2 RAM in 8 dual rank DIMMs.
My current machine only has 4Gb 800MHz DDR3 RAM, and a single quad core (core2quad) CPU.

Windows 7 x64 internal user experience benchmark shows pretty much the same figures, except for the aero estimation at "1" for the AMD machine.

I'm wondering how I can decide which machine will be better for both purposes, the main question being whether more RAM at a slower pace can perform better than less, but faster RAM.

EDIT: The main question here is not what I am going or not going to buy, I already have both of the machines, but only one can stay.

What I'm trying to find out is whether the extra RAM can compensate for the slower CPUs (and the fact they are on two sockets, which means extra context switch lags), when it comes to modern games. And the games are nothing too fancy, all I'm looking for is for COD and Assassin's Creed (and that's mostly it) not to lag when set to medium settings, with a 450GTS or 9600GT GFX card (which I already have, and need to decide where it will go).

Thanks for the virtualization pointers, but it's a bit offtopic here since I'm well aware of the different aspects of virtualization, I'm asking in this forum and not in SF because my experience with desktop/home/gaming hardware is ~5 years out of date, and that is where I'm looking for advice.

Answer

Getting virtualization and gaming into one thing is not that easy.

For Virtualization, you need a lot of CPU cores (if your workload is very CPU dependent) and a lot of RAM, in contrast to gaming where you need less cores, which on the other side need to be faster and you also don't need as much RAM as virtualization.

Concerning the RAM speed, the answer is that it doesn't matter, see this link.

To make a conclusion, if you do let's say max 2 virtual machines at a time, you better go with the fast CPU and ~8GB of RAM.
However, as your dual core opterons are probably very old, any new CPU will be faster, so the best thing would be to just buy a CPU (sandybridge quadcore) and 16GB DDR3 RAM as RAM is really really cheap these days.
With that machine you can play games(don't forget a good GPU) and do virtualization.

If you don't want to replace your current PC then keep the C2Q and buy another 16GB of RAM (4 DDR3 RAM modules).