OK, we are coming forward....
#1. I am sorry, with 512MB VRAM you have to stay low. 1024x will be good, WINDOW MODE is a must for the time being (in order to check the performance) you may be able to go higher.
The trick is easy: run in Window mode, use the mouse cursor to extend the window the way it suits you. CoD will save automaticaly the resolution to its settings.
Your VRAM is your biggest challenge, you have a MINIMUM System (because of your GPU) so minimum is what will work for you (so setting things to HIGH is taking chances).
If you want to experiment, you need a tool like GPU-Z or GPU-Observer in order to monitor your VRAM: Is it reaching its MAX, you are in trouble (=stutter).
#2. If you use WinXP then forget about high resolutions, this game needs Win7-64bit
This is the best invesment you can do (besides investing a hell of a lot of money for a good GPU).
Besides WinXP is using DX9 as far as I know...
@KG26_Alpha: I am sorry Alpha, this beats me; How will you be able to run CoD in DX10 mode on an Operating System that does not have DX10 ???? You rewrite computer history here...
#3. I do not know what expert Storebor is but he is for sure no CoD expert
What CoD does with multi core CPUs I wrote you before so I waste no more but,
as a picture is worth more than 1000 words, here is your CoD with eight cores running (4+4virtual).
This is a screenshot
3840x Desktop resolution, move sliders to the right to see the game
Look at the amount of VRAM (Memory Used) in GPU-Z, imagine how fast your GPU reaches its limit...
I hope this helps,
Happy Flying!