Infinity Ward has released the PC specs for Modern Warfare 2. We can see a slight increase in requirements compared to CoD4.

CPU: Intel Pentium 4 2.4 GHz or AMD Athlon 64 2800+ processor or any 1.8Ghz Dual Core Processor or better supported
Memory: 512MB RAM (768MB for Windows Vista)
Graphics: NVIDIA Geforce 6600 or better or ATI Radeon 9800Pro or better
Harddrive: 8GB of free hard drive space

With "Internet Connection required for activation" Activision probably wants to lower the amount of pirated copies but this type of activation never has been any problem for people who intend to crack the game.

System Requirements:

OS: Microsoft Windows XP, Windows Vista and Windows 7 (Windows 95/98/ME/2000 are unsupported)
CPU: Intel Pentium 4 3.2 GHz or AMD Athlon 64 3200+ processor or better supported
Memory: 1 GB RAM
Graphics: 256 MB NVIDIA GeForce 6600GT or better or ATI Radeon 1600XT or better
DirectX: Microsoft DirectX(R) 9.0c
Hard Drive: 12GB of free hard drive space
Sound: 100% DirectX 9.0c compatible sound card
Internet: Broadband connection required for Multiplayer Connectivity. Internet Connection required for activation.

Infinity Ward didn't mention recommended requirements though.

Battlefield Badcompany 2 • Server Patch R19 ( mappack 5 ) • New rush map for Badcompany 2 • Bad Company 2: Vietnam coming

Clan News • A New life in mAsked • CounterStrike Source Team for mAsked • New Members

Games • Wolfenstein Patch 1.11 • Minimal System Specs for MW2 • Call of Duty: World at War 1.5 patch is out !

General News • Serverbrowser: Game Scanner • A Happy New-Year !! • Merry Christmas

Misc • World At War Patch 1.2 is out ! • Happy New - Year

Trackmania • Fullspeed server again for us • Change in 1 of our servers • Trackmania client update