Look at it this way:
Card A has 10 pipelines, 500mhz/1000mhz clocks and 512MB of RAM
Card B has 14 pipelines, 700mhz/1500mhz clocks and 256MB of RAM
Card B will win everytime.
Don't get drawn in by bigger amounts of RAM on video cards - all this allows you to do is:
A) Store bigger, uncompressed textures - while this gives better quality graphics its normally only available at highest settings
B) Give you a bigger e-penis
Therefore, there is no point getting a low end 512MB card - only consider very high end ones
My advice would be to stick with your current card and buy a board with both PCI-E and a AGP slots - I believe you can get both AM2 and 939 versions, as well as Socket T, made by AsRock