Also in my personal opinion get a single card now, save up more money, and make a completely new build next year with new hardware.
Like I said previously I have a i7 960 OC'd to 4.1Ghz. A Radeon HD5970 which I sold, 12GB of RAM, a 256GB SSD, and a 1TB HDD.
This isn't the latest but it still does very well for me.
What I plan on doing currently is just getting a new card no more than $400 so I can play most games maxed out and then upgrade next year. I don't have much money right now, and since most hardware out now is fairly dated in today's terms, I will wait for the next high end Intel CPU, and couple that with new RAM, GPU(more than likely, possibly SLI maybe not), HDD, and Motherboard.
I am going to try my very best to get the best stuff out there. I am a college student and have a job, so hopefully I will be able to do it by the summer.
The reason I go high end is because I want to play all my games maxed out and not have issues with them.
My rig with my CPU being OCd can play most games out there, but some I do have slight issues with, and its usually not the vanilla version of the games. For instance Skyrim heavily modded near launch 4+ gigs of mods, ran well but not as well as the vanilla game. BF3 I can't use any form of in game Post Processing or AA. In GTAIV I can't get the ICENhancer mod to work with 30+ at all, but that could be my own fault.
What my point is, don't waste your money now on a marginal upgrade to have to do a bigger upgrade in the near future.
You have $600 now, use a good $300 or so for a nice GPU, save the rest, and build up more money to do a full upgrade like I am going to do.