Absolute Minimum will be a GTS250/9800GT
You will get 125 Frames only with no antialaising and dynamic lights disabled
A GTX260 will get you 110 frames with everything maxed right out.
Drop back to 2x AA and you will get 125.
Ive had both these cards in two seperate systems @ 1680x1050 and these are the results ive got from these cards, bear in mind the frame rates Im telling you are the baseline figures, eg. 125 is the lowest they will drop to, my gtx260 will actually go up around 170 frames but I like to find the lowest frame rate and lock the game to that figure, eg /com_maxfps 125. Gets rid of that stuttery feel IMHO.
A Good dual core is all you will need cpu wise E7500 or above, the games only threaded for 2 cores. You wont be CPU bound with an E7500. Bear in mind though while you will get 125 frames in multiplayer, singleplayer is far hungrier on CPU. I only get 60-80 frames in single player with an E8400@ 3.8Ghz.
Good low latency ddr2 at 800mhz ram wise.
The above will give you what you desire at a good price BUT you will be locking yourself into socket 775 architecture with no real upgrade path past a q9650.
In my honest opinion do yourself a favour and wait 3 months for prices on the new I5's to drop a bit, also with ATI's new cards coming out the price of there 4890's should drop even further as they runout stock. For the same price bracket as building with the above components you will pick up a system with 20-30% more grunt. With the extra power you should be able to hit 125 frames with Modern Warfare 2!
Hope the info helps. See you on the battlefield