I think you could do it reasonably with an 8 or 12 core CPU and 6GB to 8GB of RAM. Doesn't AMD have an 8 or 12 core CPU now? Not sure if Intel does yet? I think the kind of hardware you'd need to pull 2 instances off would be quite doable with domestic spec. No need for a super computer.
While AMD does have their newest Bulldozer series of processors, they are absolutely terrible in quality. The 8 core processor is only good if you MULTITASK and assign programs to use different cores. In theory it would work if you assign one instance of CiV5 to the first 4 cores, then the second instance to the last 4 cores. Although you spreading the already low quality GHz thin.
Doesn't seem a very good use of money to purchase an 8-core CPU, when buying the 2500k Sandy Bridge Intel i5, getting an H70 self-contained watercooling (Mineral oil, doesn't conduct electricity) and then overclocking the to like 5.0GHz. Then spreading workload between the 4 cores. Although this would cost considerably more. $200 for the i7, and $80ish for the cooler. The
AMD FX-8150 Zambezi 3.6GHz Socket AM3+ 125W Eight-Core Desktop Processor costs $269.99 + Tax. And you could add a Noctua Heatsink+Fan, or even a H70. I hear the overclocking potential of the FX-series(bulldozer) is very high.
I'm having a computer geek moment. Sorry everyone. Either way, the cost difference between Intel and the AMD is very close, so close it's not really a factor. I'd always grab the Intel Sandy Bridge just because you will see a higher performance increase on individual applications. If you are always multitasking and have multiple applications open, go with the 8-core monster. It's up to you. I don't multitask, at the most I'll have a firewall program, google chrome, steam, ventrilo, and then a game. I'm running an AMD 955 processor (Quite old) with 4GB of DDR2, and it is tolerable. Couldn't imagine how blazing fast it would be with a sandy bridge OC'd to 4.5GHz+
The RAM wouldn't be an issue once you get 8GB, but I'd make sure to get some real good gaming RAM with low timings. Not hard or expensive to get either.
For anyone interested in the newest CPU series by AMD, here is a chart
EDIT: 12 core CPU's are mainly for servers, and would not perform well under gaming and consumer circumstances.