So i was thinking of upgrading my GPU to an MSI Radeon 2GB 7850 Power Edition or an MSI Radeon 2GB 7850 Twin Frozr III or even an MSI Radeon 1GB 7850 Overclocked. I will be playing BF3 multiplayer (either 32 or 64 player servers) on medium or low settings and hopefully get around 35 or 45+fps at a 1920x1080 resolution. The CPU I am using is an Intel Core i3 2100. I was going to buy the 7850 Power Edition but I am worried that it will bottleneck my CPU.
No. The GPU isn't supposed to bottleneck the CPU. It's the way around. I don't think you are going to run any problem. If you're worried though, you can get a cheaper i5 models, like i5-2300. For the GPU I think you should get the 2GB RAM model, because it'll be more future proof. I'm leaning towards the MSI Power Edition Model.
Thanks for the help, what kind of a performance hit will i suffer if the CPU is bottlenecking the GPU because i was hoping to get around 45+fps, but if it is going to bottleneck in 64 player maps then would i still be able to sustain around 45+fps if i turn the settings down?
I highly doubt you will face a bottleneck. Your best bet is to get the graphics card suggested and then see what FPS you actually get. If you are only aiming for ~50 then you should be fine. Have Core Temp and CPU-Z running to see how much load is on the CPU and fraps to measure FPS. If they are all good and acceptable then happy days. If not, there is not much you can do other than upgrade.