ive been gaming seriously for about 3 years, here is what i know...
its 100% possible to play on anything above 30fps, anything below is choppy and limits your skill sevearly, 30 fps is still crap, because it not only affects performance but like in call of duty, the rate of your gun fire, how fast you see the enemy, how fast you can react to them and most importantly how smooth it is while you move, to get this you need atleast
60 fps. on my rig i get 90-200, depending on surrounding in call of duty 2. I wouldnt suggest overclocking your video card untill you try configuring with the game settings. there are lots of tweaks out there, i know one off hand that might incease performace, /snaps 30 , here is a tweak guide for cod2 that might help you with vcod.
like i said you can play on 30 fps but with difficulty, try tweaking the graphics in game before overclocking, or spend 200$ and get a good card like a 6600gt or better.
your video card is your problem, your cpu is fine but that card is outdated for the newer games... if you do try overclocking, go up slowly, (like 5-10 mgz or something) and run your game when ever you make a change, do it until you see pixels missing on the screen, (like white/black lines) then underclock it back about 20.