If Riddick challenges top end cards, I'd say that's because it's a lazy port rather than anything else. There are plenty of Xbox-derived games that run slower in PC trim than they have any right to (even bloody Halo, which started out as a pure PC title).
But I generally agree that the bulk of graphically intensive (or whatever you want to call it) games over the next couple of years will likely be Xbox2 ports. For sure there will always be one or two pure PC Doom3 or HL2 type titles that lead the way in terms of leveraging the power on offer from whatever is the prevailing high end PC tech. But the PC gaming market is just too small and too compromised by piracy (I hate Steam with a passion, but I dont blame Valve for doing it) and as ever a lot of for-PC titles will pretty low in order to enable the largest possible market etc. The cost of developing games is getting pretty crazy as the graphical quality explodes and the time taken to generate assets and whatnot similarly expands. I don't see many pure-PC titles pushing the boundaries in the short to medium term. I doubt there will be more than two or three D3 / HL2 standard (in terms of pushing the boundaries of visual quality, and let's be honest, the Source engine is nothing special, it's more the quality of the art work and assets that makes HL2 what it is graphically) PC titles in the next 2-3 years.
