Unfortunately the 775 is all but dead. You have the best 775 SLI mother board there is. Take care of it because if it breaks you will not be able to replace it. Your best bet is to get a new card and just live with the slight bottle neck for a while.
I have an Asus 750i with a Q9650 I Originally had 2 8800gt in sli but upgraded to a single gtx470. I got a huge gaming lift. Later I got a second card and saw a small increase over 1 card, and for the most part the CPU is always at 100% while the cards bounce anywhere from 50-100%. This suggests the cpu or board cannot get the video cards info fast enough. Ultimately I am still pretty happy with the system as it plays BFBC2 and COD BLOPS maxed(AA at 8x) at like 60-90 fps.
If you want a new board you can get a new 775 with a newer intel chipset that can run DDR3 1600. But that will not do SLI. So if you wanted to get a single big card like a GTX580 that might be the way to go. I don’t think this update would make sense because the ram speed increase alone would not justify the cost.
I think you should find the best single card that fits your budget and just have fun with it. My q9650 with 800 mhz ram and dual gtx470s gets slightly higher FPS than my buddies i5 2500 with a single gtx570.
I plan to wait on the next CPU platform there is alot of new stuff coming, (bulldozer PCIe 3.0) or maybe even wait a year and get a new board cpu and ram and transplant my 470s.