The HD 5770 probably doesn't even use the full bandwidth of PCI-E 1.0 so I wouldn't see any performance impacts, however, an HD 5770 can't max Crysis - you'd need something like an HD 5970 to play Crysis 1920 x 1200 0x AA on Very High. It would be worth upgrading from an 8600 GT however.
'Play' as in > 40 FPS constantly (30 possibly, some games, not sure about Crysis in this sense). I could 'run' Crysis @ 1920 x 1080 4x AA, and I could technically 'play' the game. However, it's not really playable unless it's smooth and it would probably be running at 5 FPS if I'm lucky.
That's Crysis, between 20 FPS - 30 FPS (I'm not surprised if the majority inflate their scores), 8x AA Very High, I'm not sure about resolution - it says 1280 x 1024, 1920 x 1080 & 1920 x 1200 in the tags and I can't read his settings in the intro.
This is all dependent on whether DirectX 10 is enabled or not. (DX 10 off you are right, lol)
I'm taking this from benchmarks mainly, here are the first three from Googling 'HD 5770 Review':
Not really playable in my opinion - @ TechSpot it's playable, 34 FPS, on medium settings, DX 10 enabled (1920 x 1200). @ Guru3D, it's somewhat playable, 26 FPS 1920 x 1200 with Gamer (High) and 2x AA. @ bit-tech, 1920 x 1200 4x AA 16x AF DirectX 10, High Quality is unplayable (I wouldn't say so, 23 FPS MAX). So I doubt it could 'max' Crysis (mind you Crysis Warhead's engine is a bit more optimised and runs better than the original Crysis), by maxing I mean Very High, and 4x AA, although that is debatable.
I suppose it is debatable whether 25 FPS is playable or not. For me and most others, I doubt it is really.