If you want to upgrade, your CPU is fine, just upgrade the GPU. Tom's did a test on the single player and they had it running on an i3-2100 just as well as an i7. I would think your CPU outperforms an i3.
This of course only applies to BF3 and other games that are GPU dependent. Some games are more dependent on the CPU.
What resolution do you play at?
HD4850 is good for low and middle resolution (up to 1600x900). I think you can play it med/low setting depend on what resolution do you play at...
@ OP, if you're playing at 1280 x 1024 you'll be fine even @ high quality, if you're running 1680x resolution you'll be able to get around 30 FPS @ high quality.
talking about upgrade, you'll have to upgrade your whole system.
If you want to upgrade, your CPU is fine, just upgrade the GPU. Tom's did a test on the single player and they had it running on an i3-2100 just as well as an i7. I would think your CPU outperforms an i3.
This of course only applies to BF3 and other games that are GPU dependent. Some games are more dependent on the CPU.