What is the best AGP card I can get?

I have an Alienware Extreme with an GEFORCE 7800 W/128 ON MEMORY i WANT TO STEP UP AND GET ONE WITH AT LEAST 512 OR EVEN 1 GB. mY OLD dELL xps HAD AN 8800 pci EXPRESS CARD. BUT THIS HAS JUST AN agp SLOT.Any sugestions? I play the latest games. COD Modern Warfare/ plus all COD games. Medal of Honor All of them. Just want the best card I can for the max resolution. the 7800 works great., just need more memory in the card.