Hello,
I've Radeon HD3650 1GB ddr2 and gtx 550 ti 1gb ddr3. Could I connect them together on my PC with a motehr board gigabye x79-ud3 and a cpu I7 3820 and 16gb ram?
Thanks in advance

No. So far only cards with the same chipset and similar specifications can be used in sli or crossfire. Since the Radeon uses crossfire and the gtx sli they are completely incompatible for bridging. If you try to use them at the same time your asking for trouble since the drivers for each are different and will probably conflict during boot and your PC will not start. I have heard that nVidia is working on (maybe already has) a way to join two different cards as long as they can use the same drivers but its something new and would not include Radeon in any event.