Honestly I don't get your point. Since you are playing it with a 6800, you know that your FPS can drop to 18-25 fps if you fight outdoors with 2-3 enemies, nothing else going on. No HDR and at medium settings. An xBox360 will do the same scene at 35 FPS with HDR and higher res for a package price of $450.
When FEAR 360 comes out it will render better than your machine could the original game. You will not be able to play Crysis with decent FPS without an upgrade. The xBox360 will do Lost Planet and Gears of War with no upgrade. Meanwhile, you'll be getting ready for DX10 and Vista, possibly a new CPU, MB and DDR2. Its a neverending cycle of upgrades just to keep the mouse and keyboard. The next console "upgrade" will again only be the cost of a vid card.
All I'm saying is that given the 360 lineup looking this good its getting hard to justify holding out for a keyboard + Mouse for all the extra cost.