A 6600 can run Half Life 2 just fine. HL2 easily competes with console games, and does not look like crap.
The thing I see here is, is that HL2 was designed with lower-spec machines in mind, and setting a performance budget for each portion of the game which is cant exceed to keep the game from getting jumpy...
Regarding Gears of War, I blame the fact they designed the game with the 360 specs in mind, and no sense of a fall-back for machines slower then that..., it's their fault they didn't plan ahead for a scalable PC version.
Regarding the slight bit of rant, dont PC gamers of that flavour playing PC-intensitive games of that sort usually have dedicated modern vid cards? Or am I just dreaming here...
(not ones capable of fully utilizing Unreal3 engine of course, but you know, good enough for modern games at good detail)