Operating System
You'll need the 64-bit version of Windows 7 in order to make use of 4GB of RAM. 32-bit operating systems only recognize up to 2GB of RAM, therefore the extra two you're looking at would be wasted.
CPU
I'm not sure why you're asking about 2.93GHz when your list states 2.66GHz. Regardless, nearly all computers are dual or quad core. I don't think dual or quad core make much of a difference in games. You'll see games that list dual core as a requirement, but as I said, practically all CPUs today are dual or quad core. Listing a single core might confuse consumers.
Where dual or quad core make a real difference is within the OS itself and using different applications. The more programs you have running (especially those in the background, like virus scanners and software firewalls) the more the CPU has to do. In a dual or quad core processor, the tasks become more evenly divided so more can be done at once without a drop in performance.
As an aside, it's really difficult to find a single core processor. My computer has a single core processor, but I got it in 2004. The CPU is 3.2GHz and even though it's single core, it has Hyper-Threading which emulates a dual core to some degree. I also have 2GB of RAM and a 512MB ATI Radeon 3870 (the computer originally had a 256MG Radeon X800 XT). Where my machine is really lacking is that it only has a 250GB hard drive, but I could replace that if I wanted to. I'm simply too lazy to do it and I've never managed to fill it up, so why bother?
GPU
It's hard to tell which is the best video card. Statistics on paper may differ from actual performance. Plus there are many other variables involved.
It looks like you're going for a Crossfire setup (dual ATI video cards) which is really good. Unless you're getting two 1GB video cards, I'd say you're better off getting one 1GB card, as opposed to two 512MB cards.
There's a huge (and ongoing) fight between ATI and NVIDIA fanboys. Unfortunately, it's not that simple. Each video card is different, no matter who made it. There are also many variants of the same card. It's nearly impossible to state which company is "the best."
Having said that, the general consensus is that NVIDIA makes faster cards, while ATI has more stable cards. Some people might disagree with me, but in my experience, NVIDIA cards have always been more temperamental. In other words, they seem to have more issues (Maia's video card was an NVIDIA GeForce 8800GT and it fried itself, requiring her to get a replacement) and updating drivers on an NVIDIA card has been more difficult, as well.
Once again, some people will disagree with me. They'll say that "NVIDIA video cards are super-stable and updating drivers is a cinch!" Well, good for you if that's your experience, but I haven't been so lucky.
I've used ATI video cards for a while, not because these cards are necessarily the fastest (although speed once again depends on which card you're talking about), but due to the overall stability. ATI cards seem easier to update drivers on and new drivers come out about every month.
For the most part, the differences between ATI and NVIDIA cards is minimal. Maia has an NIVIDIA card and I have an ATI card, but they're about equal in performance. Some games work better with her card and some with mine, although it's hard to spot this unless you're testing the frame rate. Simply by looking at the game and judging how it plays, we seem to have the same, or at least similar, experience.
