well i since 7950 have a close performance to a gtx 580 normally the relative performance is 1-2% better, and 560 ti in sli is >14% better than 580 (in some games) i think 2x 560 ti still better than only one 7950.
i think 560 ti in sli can be even better than a 7970 but very close performance.
EDIT: yes a single card is more trustable than going for SLI or CrossFire but if you wan't a single card to get close performance take 7970 instead of 7950.
Yes agreed, SLI needs a more powerfull final system, better motherboard, better PSU, and also you will need to keep updating drivers/etc fixing normal SLI issues, but 1 7950 is not better than SLI 560 ti, but yes i always will recommend a single card solution than SLI/Crossfire.
I'm just considering the option cause the two gtx 560s are cheaper than a single 7950. And no I am not willing to wait for a price drop since this is my first rig and I've been waiting to build one since like, forever.
You would be wasting nothing. Why not go for a single high end card and buy another one just like it when you can afford?
I bet two 7950 or whatever you want to buy(in case you want to wait for Nvidia's new generation), will be much better than two GTX560 ti. If you already have/had 1 GTX560 TI i would recommend buying another if it would fit better in your budget but like this i would not recommend at all that solution.
Also there is the issue of micro stuttering in games when using SLI or crossfire. That problem is less encountered when the video cards are more powerful(high end).
get the hd 7950 single now or wait for kepler because the newer cards tech is increasing day by day that we can see for example look at the hd 7750 it is powerful like an hd 6770 and it uses less power then hd 6670 so you should wait for newer series cards or just buy now the single high end card hd 7950 you will be happy with that upgrade and when you feel like its time is gone then crossfire or go for another single high end card and sell it.