Yes, your Athlon II X3, while a very good processor, is the main bottle neck for your HD 7950, especially with Metro 2033. The card's GPU is waiting for the CPU to catch up with it in demanding games like that one. Metro 2033 is a very demanding game graphically. Especially at 1920x1080.
Having said that, 45 fps is quite good frame rate for that game. But it is all due to the graphic card. With a faster CPU, you will see improvement. But there are also other factors... the hypertransport buss speed, the lack of an L3 cache, etc.
If you look through majority of game benchmarks, the II x 3 can hold it own to provide playable framerates except in multiplayer games. While it is limiting the full potential of your 7950, in Metro 2033 specifically, the GPU and the CPU are working near their max already. If you look at the benchmarks I posted, In Metro 2033 you will not gain too much FPS by going to a new CPU since the bottleneck is the GPU. In other games of course the CPU is going to hold you back.
Interesting article. I'm going to have to read the article this one was a follow up on next. Explains in easy to understand terms why high frame rates can still look choppy at times. Frame rate is mainly dependent on the GPU, but smoothness is dependent on the CPU.
I would say that even if you are doing okay with your FPS during gameplay with your Athlon, in the interest of simply using the computer for everything else up to opening the game, it's time to upgrade. You have the 7th fastest GPU to hit the market as of today- meanwhile, the benchmark chart shows your CPU as the 52nd fastest among just AM3/AM3+ chips!
It sounds like you already made the decision- are you doing a fresh Intel build and just keeping the card/psu/case?