:::URGENT::: Graphics Card Help?

I want to buy Call of Duty 4. I surely know that my current Graphics card won't even get near 30 Frames Per Second.
I have come here to see what Graphics card I can get of at least 300$ that will deliver Maximum Performance.
These are my Computer Specs.
Processor: AMD Athlon 64 X2 Dual-Core Proccesor 4400+
Memory: 2048
Graphics Card: NVIDEA GeForce 7300LE
The graphics card manufacturer set the cards Engine clock to 450
But since I really wanted better performance, I downloaded Powerstrip and started overclocking the card all the way up to 550MHz
The Memory Clock (at manufacturer settings) was on 532MHz and since me being so curious, I set it to 650MHz
Hard Drive: 320GB

So I come to this question, What Graphics card is one that is Xtr33mly fast and will be able to be handled by my computer?(HP Pavilion m7580n)
I intend to mess around with every single little tiny setting in my computer and in the game (Counter-Strike:Source) I get around 100+ Frames Per Second because of my settings.
Thank you in advance