I'm confused, that's all, lol.
How much FPS would I get for what you suggested?
Same.
Alright.
Let's break this down.
You have a stuff CPU.
The graphics card information has to redirect through the CPU, before it gets to the graphics card. This means that what performance you get with really good graphics cards is entirely down to how much your CPU can process and 'let through' to the graphics card. Since both the HD6790 I suggested and the HD6850 Cyanian suggested are really good cards, they will both give you the same performance with this current CPU.
If you were to buy the HD6790 and then later on get a better CPU, let's say a Phenom II, then you will get more FPS in BF3 than with the old CPU, but not that many more.
If you were to buy the HD6850 and then later on get the Phenom II, then you will also get more FPS in BF3 than with the old CPU, but this time it will be considerably more, as it's a better card.
So the entirety of how well your GPU does relies on your CPU, which isn't very good.
But both cards will give you playable FPS on reasonably high detail right now.