Well I'm going to concur with Pinoeerisloud regarding either one GTX 680 or AMD 7970 for a couple of reasons.
One is that a powerful single card solution is much easier rather than deal with crossfire or SLI for multiple reasons.
Heat that may restrict full potential over clocking. Micro stutter. Driver issues or incompatibility for dual GPU in games that aren't supported or lack of good support that can lead to frustration.
Another is price obviously. What you'll find is either 7970 or 680 are so much better than previous gen cards that you'll get almost as good performance in some areas close to or above a 590 or 6990 dual GPU on single pcb.
Notice the synthetic benchmark below how close the 680 and 7970 is getting to one 590 ( which is bascially two watered down 580's) and one 6990 (which is bascially two 6970's).Legit Reviews - SourceBenchmark Reviews - Source
Now keep in mind this is just a couple of graphs and you should view many from different sources to compare.
There are some great reviews you should consider reading to familarize yourself with the GPU your thinking of buying and it's potential. Obviously you'll want to stay focused on the specs of 2560x1600
resolution that pertains to you.
Here is some of the more recent review sites that discuss both newest cards.HardOCP
- Hardware Heaven
You'll find starting with either the 7970 or 680 a better avenue to take off the bat. It will let you set a little money aside in your budget for something else and give you comparable performance. You also leave the door open for later down the road after your set up is finished to eventually get that second GPU to tread into the dual GPU field.
I find I end up going with the next single powerful GPU every year over going dual. My reasons are different though because I'm about to build a second rig for myself so I can hand down this one to the family room and hand down my used GPU's now. Each person has different reasons why they'd choose dual or single. In my opinion as mentioned prior a single 7970 or 680 is your best bet.
Either card in a single GPU solution is a strong card and there isn't really any wrong choice. These two series really did a great job all around for many different reasons. AMD is now rendering tessellation well and Nvidia finally can render multiple monitors with one GPU as well as AMD. Both do great in gaming and you won't be disappointed over one of these rather than a dual GPU set up of any other variant.
When you finally decide which card best suits your needs I'd join either of the clubs and follow them. You'll gain a lot of knowledge just by reading what others are expericneing with their cards and if you should choose one it's a great place to ask fellow members that will have much more insight.[Official] AMD Radeon HD 7950/7970 Owners Thread[Official] NVIDIA GTX680 Owner's Club
[If you go to either of the club threads and post this question 'which card?" they will both tell you their own clubs card.
Good luck in your endavoar and welcome to OCN.