Should be alright. Even in SLI I don't think your power draw will be more then 500W, and you have 780W on that PSU. You'll be fine.
^+1 650w would be enough for two of those cards. But for $10.00 more you can get the GTX 560 Tis which are a better card then the just the 560. Now if you are going to run resolution higher than 1920x1200 or more than one monitor then the 2GB of Vram you will need.
If not then just go with the 1GB of Vram. But yes eveything that you linked us to will work as long as your MOBO will support SLI. I do wish you good luck on this.
I agree with DM186. The Corsair TX 650 should very well be enough for SLi GTX 560 Tis.
And it should save you some cash to buy the 'Ti' cards, instead of the non-Ti ones.
Your PSU can even handle a gtx 680 in sli, so yeah go ahead, it is more than enough
The GS800 should be able to handle the GTX480 in SLI, not the Corsair 650. The GTX480 is rated at 250W each, while the GTX680 is rated at 175W. (same as the GTX560TI.)
Thx guys and gals thx to the comments im going to get the 1gb to save money since im on a single screen, and get a 650 psu to save aswell, as far as the TI suggestions and my mobo being able to sli currently it cant its a crappy stock mobo with 1 16 pci-e and a length limit on the gpus of 8.25", but in the future i plan to buy a new mobo that has 4 pci-e slots and plenty of room for long graphics not to inter fear with the plugs.
I have the Gs800 running 2 gtx 480's at stock speeds, i have ran furmark for a hour while stressing CPU with Linx, only pulling 663w from the wall matey.