Memory Compatibility-Please help me


Beta member
I have a new dell dimension 8400. I ordered what I was told was compatible ram with my system. The ram is Corsair CMX512- 3200pc2 400mhz, 184 pin. I bought this because I read it is the best memory to buy for high performance gaming and I would like to use my new copy of Battlefield 2 to its full potential. Got it today and it wont fit in the slots! Do I need 240 pin? Someone else told me now that I need 533mhz? but the ram that came in my computer is 400mhz so I dont know... any assistance is much appriciated and I can answer any questions you have about my pc to the best of my ability. Thanks a lot guys.