I'm looking to pick up a used computer. It is a Phenom 955 + 4 gigs of ram + Crossfire 2x Radeon HD 4770.
How would this Crossfire combo compare to a single video card?
Here's another article from Anandtech.com that compares the performance of one vs multiple GPUs.
You ask to compare it to a single GPU, which is a faulty comparison, since there are also GPUs that have two graphical chips on one PCB. It's one card, but has a much better performance (and off course costs a lot more) and is often recognizable by the X2 label.
However, I assume you want to know how much SLI/Crossfire will scale, regardless of the GPU model your using. Here the answer again is: it depends.
Not every game is made equal, so some games will have near perfect scaling with additional GPUs while others have marginal gains. The same goes for GPUs, not every architecture has the same scaling (though it's getting better nowadays), not every card has the same amount of RAM (or even the same type DDR3 vs DDR5) and new architectures generally perform better than older ones (if the specs are comparable).
I would suggest checking benchmarks to see what performance a said GPU has in the game of your preference and buy that one.
Here's an example for Crysis Warhead:
As you can see performance comes at a price, so it's up to you to decide what performance you want and picks the configuration with the best bang for the buck that gives that performance. If need be, you can always decide to upgrade by adding more. Note: make sure your PSU can handle more cards and that your case is large enough to fit more cards
It depends on the single video card you are comparing the CrossFire combo against. In some scenarios, a single card could outperform a CrossFire/SLI setup. Here is a good post that explains in more detail: