The person above posted a really nice build. i3 2100 is indeed a great gaming CPU, but as you plan playing BF3 when it comes out I'm unsure that the i3 can play it without any hiccups as its expected that BF3 will be able to utilise 4 cores and 6-8 threads.
So, an alternative AMD Build :
Processor : AMD Phenom II X4 955 BE - $120
MoBo : A good ASUS or ASROCK AM3+ MoBo - $80
Graphics Card : Sapphire Raedon HD 6850 - $150
PSU : Corsair CX500 - $50
RAM : 1x4GB Corsair or Gskill DDR3 1333 - $35