For gaming, I would go with 4GB of RAM without a question.
You don't need two graphics cards at all. A lot of the time, SLI (Nvidia's dual card feature) and Crossfire (ATI's dual card feature) are a big waste of money unless you are playing games at extremely high resolutions. Even then, you can get a single card that offers close to the same peformance.
As far as processors go, most gamers today go with either a quad core or a dual core. Single core processors (Pentium 4, etc...) are a thing of the past for gaming. However, most games don't fully take advantage of quad core CPUs.
This leads to an issue when picking a dual core versus a quad core. Effectively, you have to pay more per core with a quad core CPU. For example, a quad core CPU with the same speed as a dual core CPU is significantly more expensive, since it's has four cores. This is well worth it if you play games that take advantage of quad core CPUs. Yet if the game you want to play doesn't
take advantage of quad core architecture (ie - most of the games out now), then you're going to get the same performance you would have gotten with the dual core CPU, making those extra cores a waste of money.
For example, if you are spending $300 on a CPU, and you play games that don't support quad cores, then you would want to go with the $300 dual core over the $300 quad core, as you would get better performance.
With that said, there are probably going to be more games coming out within the next year that take advantage of quad cores, so you have to keep that in mind. It really comes down to what games you are desiring to play.
Keyboard, mouse, and monitors are all relatively personal preferences. I'd recommend a 22 inch widescreen monitor though. Samsung usually puts out grat LCD monitors these days.
I know might sound daunting, but building your own PC isn't difficult at all, and you can save a lot too. It's really just like building with LEGOS,
. There are tons of helpful guides and forums out there (Anandtech, [H]ardOCP). Plus, if you put you're own quality time into it, you'll probably encounter less stability problems, whereas a kid in some sweatshop in China slaps together a hundred computers a day without putting quality work into it. I built my first computer three years ago, and I've yet to encounter any problems whatsoever. It's been rock solid.