Minimum MW2 System Requirements
The minimum system requirements for the PC version of Modern Warfare 2 have been released:
||Microsoft Windows XP, Windows Vista and Windows 7 (Windows 95/98/ME/2000 are unsupported)
||Intel Pentium 4 3.2 GHz or AMD Athlon 64 3200+ processor or better supported
||1 GB RAM
||256 MB NVIDIA GeForce 6600GT or better or ATI Radeon 1600XT or better
||Microsoft DirectX(R) 9.0c
||12GB of free hard drive space
||100% DirectX 9.0c compatible sound card
||Broadband connection required for Multiplayer Connectivity. Internet Connection required for activation.
The system requirements seem to be almost identical to Call of Duty 4, so if you are able to run that with decent frames, you should have no problem running MW2 on your PC. The recommended system requirements should be available soon.
As a side note, it seems that the game requires you to have an internet connection to activate the game. Hardly surprising, since MW2 has been partly integrated with Steam.