Easily checked:
Run the Task Manager (CTRL-ALT-DEL) while a game is running. Close out the game and look at your CPU history.
Is one or more cores maxed out at 100%? You're CPU limited.
Considering my X2-4800+ S939 is running an average of 75% but up to 90% and occasionally 100% on one of the cores using a "lowly" HD3870 I'd say you are definitely bottlenecked. I figure the HD4770 would, on average be the best card for my X2-4800+. I'll probably get one to tide me over until I get a 6-core Intel and DX11, SATA600 etc.
You still have a pretty nice system for many games. This is what I'd do:
1) overclock the CPU by 10% or more (if needed, careful to drop RAM timings first etc)
2) replace the system in at least another year
At this point in the PC hardware world, if you have a half-decent PC I recommend waiting for SATA 600 motherboards and DX11 graphics.
Lots of games will run awesome on your system. Sure, it's no Crysis killer. I can say Diablo 3 and Starcraft 2 as newer games will run really great.
So, yeah, your CPU-bottlnecked. I say pick games that run good on YOUR system and don't feel compelled to upgrade constantly.