I only used that as an example - I should have expanded on things a bit more. I used that only as a reference because most people can relate to it. Yes, it totally depends on the game and the resolution you want to run at. However, the OP's question was simply is his processor bottlenecking his video card. In simple terms, yes, it is. Metro will use multiple cores - the others, not so much (at least not that I'm aware of, but I could be wrong - wouldn't be the first time). He could get much better FPS in most games, but not all as you stated.
But I COMPLETELY agree with you on not worrying about it unless there's some game that just isn't cutting it anymore. If you're happy with your system and you're not noticing any slowdowns or stuttering, then don't worry about it. Yes, a processor upgrade to a quad-core would help in most games, but not all. Is it kind of silly to spend $200-$290 on a 9550/9650 - probably. But, that's still a cheaper option than an i750, a good 1156 motherboard and new DDR3 memory, not to mention he wouldn't have to reinstall Windows. And, in 90% of the games out there, the jump to quad-core and to Penryn (from Conroe) would allow him to really push the 5770.
Just my 2¢