I have used an athlon X3 in a number of builds, and it is a good little performer. i am not sure how it would do in BF3, but in terms of skyrim, swtor, and the mass effect three demo, it has handled most games quite well.
That bring said i am not sure how much of a gain you will see over an x2. This and they are no longer producing the chip so they are harder and harder to come by at reasonable prices, but all in all they are nice little CPUs, i was able to overclock a few on stock voltages to around 3.6 ghz, the other to 3.5.
But you might consider a phenom 2 if i were you, they have an l3 cache and more cores usually. The 945 is usually cheap when you can find it. I am guessing you have an AM3 Mobo right?