Well how many? 10 or 12-15?
My PC that I designed last summer to be 10-box capable uses a GTX 570, a 4-core+HT i7 (forget the model number off the top of my head, sorry), and 16GB of RAM. You also want a fast hard drive, and loading times would quite likely benefit from an SSD. You're also going to produce a lot of heat; my case is one of the High-Airflow cases from ThermalTake -- I'm not a fan of watercooling and I wanted something quiet this time around. Gigantic-ass fans on this thing mean quiet
4 vs 6 CPU cores is going to be a HUGE difference if you're going 12-15. It's easy to explain why... 12 divided by 6 is 2, and 12 divided by 4 is 3. That's the number of game instances that would be handled by each core, and the lower that number is, the better performance you're going to get (probably as it approaches 1; going lower than 1 is less likely to be a huge benefit).
As far as video card, if you're talking 640x480 and 800x600 with medium/low settings, then sure just about the only metric you're going to worry about with a current-gen (or recent) mid or high end video card is the amount of RAM on your video card. That's some low resolution though
I think mine run at 1920x1080.
The amount of RAM on your video card is going to be important to how many game instances you can launch. There needs to be enough remaining in order to allocate the backbuffer(s) or the game will fail to launch and produce a DirectX error (e.g. can't start 3d, etc). Yes, of course it also stores textures and such, but most of those can remain in system memory if the VRAM is consumed (or you might start seeing missing textures, replaced by solid colors instead). 2GB of RAM on your video card should be fine