I'd say that your CPU is the biggest bottleneck of all, the 1 Gb of RAM the second;
- get a dual core: COD4 likes cores. A X2 4200 should give you a huge boost for a small price.
- if you have a single 1 Gb stick of DDR, add a second - and enable dual channel. Second boost cometh. If not, add/replace with a new pair of DDR sticks (DDR is not much more costly than DDR2, and since DDR3 is around the corner, I'd advise you to wait DDR2 out)
Then, cleaning up will remove overhead:
- disable Theme service and sounds,
- remove as many taskbar icons and related softwares as you can,
- use a light antivirus like AVG instead of Norton,
- disable spooler service if you don't have a printer,
- disable Microsoft file sharing and Server service if you don't share folders between different computers,
- disable system restore,
- use Pagedefrag and JKdefrag,
- remove unneeded peripherals like COM and Serial ports in BIOS...
This helps make your system nimble enough to handle pretty much anything - for zero cost.