You can run your games on it but you will not get any benefit over a single processor motherboard. Multiple cpu's are great for multitasking, especially when running multiple virtual machines and programming and video editing/compiling and stuff like that as well as server boards but no game today use more than 4 cores. And those are few. With the right combination of board and processor you can run any game on the market today - and those to come for a while.
If you're willing to give up some things the current sandy bridge is ok. If you want full performance you will need the socket 1156 or better, 1366 i7's. Especially if you want to run those cards, or more at the dual/triple X16 slots. CPU comparisons are great for figuring out if your processor is capable and comparable in price. Once you have that information you can decide which is best for you and put that into a machine that has the other components you want. Just make sure the other components don't hold you back.
Don't get me wrong, you can run those 2 580's, (@ X8 X8 speeds) on a sandy bridge board and the processing power of the cpu is awesome, (and overclocks better giving you more GHz to play with), enough to run what you want, (given you get a P series board and the 2600K processor), which will also allow you to overclock the memory a little over the 1333 speed. But they are still everyday chips and not the enthusiasts chips of before. If you wait, sandy bridge has more coming out that will probably blow the socks off what is currently on the market today.
But getting back to your question - "2x xenon x5650 processors as a base for a gaming rig would give a considerable preformance boost over the latest I7 based boxes" - I believe to be a false premise.