The i3-2100 is going to outperform the FX-4100. It's up to you, though.
I have an MSI Twin Frozr II 2GB 6950 on a Z68 motherboard with an i5-2500K (nothing is overclocked). I use a 1600x900 monitor. I get an average of about 45fps and a minimum around 35fps in BF3 on ultra settings, so I guess that's about what you should expect from your build. If you're gaming at a higher resolution you won't be able to use ultra. I know some people think that a low of 35fps is too low but I honestly can't tell the difference.
Now all that being said - you're not going to do any better than a 6950 on your budget.
Also, a 1GB 6950 is going to perform the same as a 2GB 6950 (the 1GB card actually performs marginally better in some instances). If you end up getting the MIR then your XFX card is a good deal, but that's a coin flip at best. If you want to lower your initial costs then the Gigabyte 1GB 6950 with a factory overclock is one of the best cards you can get for $240 (free shipping and the free game included). http://www.newegg.com/Product/Product.aspx?Item=N82E168...