Before I build my X58 rig I had a M-ATX board, my E5420, and a GTX 460. That rig played BF3 on medium/high settings, CS:GO maxed out, and BioShock Infinite on high no problem at 1080p. I did have frame drops that don't happen with my X58 system, but my CPU was only running at 2.5GHz so that held me back. An overclocked X5450/X5460 should be equivalent to an i5 giving a superb gaming experience.
I had to sell off all of my extra hardware, but when I get my car fixed and have some extra cash I am going to get a high end 775 board and a good clocking X series Xeon and run a bunch of benchmarks. All the people who keep saying 775 is dead are going to be stunned that our $50 CPU's can keep up with their $200 CPU's.