There are technologies called
Scalable Link Interface (SLI) and
CrossFireX, just in case you guys have never heard about them

If you have never had a chance to play Metro 2033 at 2560x1600 max, wait for price drop of 30" displays and grab a couple of GTX580s or HD6970s when 28nm cards are released (and of course 28nm cards sound better if they are not too expensive).
By the way, not every game supports
split-frame-rendering, therefore if taken
alternate-frame-rendering into account, multiple cards do not simply add up the vram but instead the effective vram is the same as a single GPU, e.g. GTX590 would have only 1.5GB vram effective, as well as 580 1.5GB x 4 SLI would also give unplayable fps for Metro 2033 at 2560x1600. True experience from a user who has ever owned and played more than ten GTX580s and been in
3DMark Hall of Fame for a long while. I think he knows how to tame games like Metro 2033 better than you do, if you never had the experience. Believe it or not.