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Yes your rig can run the card, and 4GB is absolutely plenty for FHD display, you'd need tri 1440p if you want to utilize 6 or 8GB of VRAM, even in SLI. GTX 980 is a great card, 170W TDP will run with ease if your 560 Ti runs well. I don't really know any better single GPU card. Feel free to upgrade, and I'd also upgrade PSU when you plan to SLI, as Thermaltake can not be trusted for high loads.
Yes your rig can run the card, and 4GB is absolutely plenty for FHD display, you'd need tri 1440p if you want to utilize 6 or 8GB of VRAM, even in SLI. GTX 980 is a great card, 170W TDP will run with ease if your 560 Ti runs well. I don't really know any better single GPU card. Feel free to upgrade, and I'd also upgrade PSU when you plan to SLI, as Thermaltake can not be trusted for high loads.
 
Solution
Problem: Your motherboard does not support SLI

Consider: Your monitor is only 60Hz and getting a 980 will not provide any noticeable benefit over a 970 at 1080p. Also you seriously don't need 8GB of VRAM. The new 8GB cards that come out are going to be really expensive.

Apart from that the new card will work fine on your PC.