Hmmm.... Okay well there's a reason it is called Virtual Memory, and that is because it does not exist. I'm not really sure how changing the amount of virtual memory would help at all though. So I did a little looking around and here's my theory:
Increasing your Virtual Memory allows you to run more memory intensive applications, such as COD4, but the trade off to this is it is much slower. And I wouldn't recommend doing this because the side effect is it creates fragmentation. This may be good for a couple times, this is because Windows is probably accessing the new memory space you just created which is completely clear, but as you continue to do this that space will also become cluttered as Windows attempts to reuse old memory. So unless you plan on defragmenting your hard-drive every week or more (depending on how often you play COD4) then this is not a good idea.
However there are ways to drastically increase the speed of your system. One way to do so is to restart your computer by turning off all nonessential applications:
Just:
Start>Run
type "msconfig" into Run and hit OK
A window will pop up, You need Admin rights.
Go to the "Services" tab
Select "Hide all microsoft Services" > Then Click "Disable All"> Click OK> and restart your computer.Now your computer will restart with only only essentially processes running in the background.
Also: you can go to your TaskManager once you start COD4 and select its process and give it High Priority, which will give it even more processing time.
Lastly:
If you using a laptop, simply buying a cooling stand for around $20.00 can help you immensely when running intense applications. Laptobs have a tendency to over heat which is often why the are not good for gaming, but a cooling stand keeps it from heating up to much which allows you to run more intense applications for a longer time, while preserving the life of your computer.
Doing all these things I was able to run MW2 on the highest settings on my laptop, which only has 3GB of Ram!
Lol. though it was rather laggy... but I'm able to play on normal settings now without any detectable lag, except on a few large maps.
Hope this helps you.