yep, pretty much most recommended size for it is 256mb, thats mainly for resolutions like 1024x768.
then 512mb is mainly for resolutions around 1680 x 1050.
If you plan on using vista, a 256 mb will be enough no matter what resolution you have as long as you have 3gb of physical memory because vista takes 767mb worth of physical to make it share, so when you play a game and needs more than what your vram has, it will only take what it needs.
The amount is vram is for resolutions.
but i dont know whats the difference between ddr3 and ddr5, maybe its speed?