Max Video Card for Dell Optiplex GX620?

I am very computer literate, and I cannot get a straight answer on this. I own a Dell optiplex 620 and Ive upgraded it to 2 gigs of RAM and I'm running a Nvidea GeForce 8600 VC, but for the life of me I dont understand the whole DDR2, DDR3, DDR4, DDR5, DDR$$%#^# BS. I Need to know the best video card I can get in this computer without making huge modifications to the mother board or for that matter, anyother part of the computer,