Yes Dakota, the system you describe WILL definitely be able to run COD4 absolutely maxxed out, with some to spare! However, a quad, 4GB RAM and a GTX260 are overkill for a game that will run on max settings on a 22 inch screen on a 2.5GHz Dual Core and a 4670 (as on my friend's).
Adriandude, just been through your profile and found your system specs:
1 Machine:HP Pavilion a720n
OS:Windows Xp SP3
HDD:200 GB
Ram:512MB SD DDMIN PC-2700
Proc: AMD Athlon 3200+ 2.20GHz 512KB Cache
Video: ATI Radeon 7500 128MB
2 Machine: emachines D620
OS: Windows Vista Home Basic SP2
HDD:160GB
Ram:2GB PC-5700
Proc: AMD Athlon 64-Bit 1.60GHz 512KB Cache
Video:256MB
3: Machine: Dell Optiplex GX-150
OS:Ubuntu 9.10
HDD:20GB
Ram:512MB
Proc: Pentium III 988MHz
Video:4MB
4: Machine: Dell Optiplex GX-280
HDD: Primary:80GB Seconday:40GB
Ram:1.75GB
Proc: Pentium 4 2.80GHz
Video:8MB
The answer is:
PC 1 - No, your CPU might be OK, but your RAM isn't enough for COD4 and the GPU isn't anywhere close to enough.
PC 2 - CPU could cause you problems, but I think the integrated graphics on that laptop won't be up to it anyway. Radeon X1200 by the look of it - not gonna work, sorry.
PC 3 - Bless - used to use Optiplex's at my old work place. Decent office machines, gaming? Not a chance!
PC 4 - No GPU, no hope. Is there a graphics card slot in there we could put a decent £50 card in? Might have a shot.
Short version - you'd need to do some serious upgrading work on any of them to be able to run COD4 at anything more than "looks horrible" settings at a very low FPS. Sorry mate, but you don't have a gaming machine in your collection.