You won't bottleneck GTX 580's in 3 or 4 way SLI with an i7 920. The 920 is a monster still to this day. It's easily overclockable for big performance gains. Took me 5 minutes to get mine from 2.66 to 4.0 for basically a 50% jump in performance. As mentioned above, the problem for you is you'll need a socket 1366 motherboard and triple channel memory.
If you're going to do that, some (myself included) would probably almost push you to buy an i5 2500K with a socket 1155 board for 3 main reasons. 1) You could reuse your dual channel memory on that board. 2) 1155 boards are cheaper than 1366 boards. 3) 2500K > 920 for gaming purposes. I'm not sure what he's selling it for, but you can pick up a brand new 2500K at Microcenter for $180. The 2500K is also a GREAT overclocker. Much better than the 920.
To sum it up, depending on what your buddy is selling the 920 for, I'd bet that upgrading to a 2500K and 1155 board would be better for gaming, use less power, and be cheaper than upgrading to a 920, 1366 board, and triple channel memory.