you will need a new PSU also your 9850BE will bottleneck a GTX 560 you already arnt getting full preformance from a GTX 260 SLI. Gigabyte has a bios update for AM3 CPU's.
Your board with the newest bios will support the AMD PHenom II X6 1090T and lower, Phenom II x4 965 and lower, I would highly suggest getting at least a Phenom II x4 965 if you want to take advtange of dual GTX 560's, or go for the 1090T. If you leave your 9850BE you will be bottlenecked severly even at extreme resolutions, your CPU simply wont be able to feed enough data
I think I will upgrade my CPU, the Phenom II x4 965 looks good. I found an interesting article about the power consumption for the GTX 560 SLI
http://guru3d.com/article/geforce-gtx-560-ti-sli-review/14. According to that I MAY be able to squeak by with the 750W. What do you think?