PS3 + PC = Pretty much nothing is unplayable. Outside Halo 3, ODST, Reach, Gears 2, and Forza 2 & 3, there really isn't any true 360 exclusive games. L4D, Crackdown, Fable 2 & 3, Gears 1, Halo 1 & 2, and others are on PC.
I say keep the PS3, get the PC version of your 360 games, and sell the Xbox. The PC versions have better graphics, free DLC (At least Valve game DLC is free on PC, costs money on Live), and most importantly mods. It is the setup I have and there really isn't any game I want to play but can't (besides Star Fox, because I don't have an N64 or SNES).
Then again, I am biased. My laptop didn't have Vista for very long, it got booted out in exchange for Linux. Plus I am not a fan of FPS or cover based (CB)TPS games with no other elements (Uncharted at least has the platform-puzzle bits to water down the tequila that is CBTPS gameplay.