Need help deciding, please!

Guys, I need help. After 5 years I'm getting back into PC gaming (my income is substantially higher than my college years, thank god). I bought most of my components today, based off another thread in system builds section of the forum. Here's the gist of what I ordered today:

Monitor: Dell Ultrasharp 30" 3008WFP
CPU: Core i7 920 2.66
MB: ASRock x58 extreme
Ram: 6 gb pc1600
PSU: 750w Corsair

I'm looking to mainly game on the computer with some photo editing on the side (hence the ridiculously expensive monitor). I know I'll need some horsepower to power the monitor at max resolution. I play mostly FPS and RTS. I'm leaning towards a new 5870 if I can find one, and just add another in crossfire later on when one just doesn't cut it. Looking at a budget of $400. What's a good match for the build/monitor?

Gotcha, so it sounds like 5870 is best bang for the buck given my needs. You wouldn't recommend like 2 x 4890's or something like that. Probably wouldn't be very 'future proof' I'd imagine...