The RAM on the graphics card is for performance of graphics.
One thing to take on board though is that if you for instance had 128mb of system RAM and bought a 256mb Graphics card it wouldn't be much cop performance wise as you would get dogged down by lack of performace by the system you need a good balance of both...
basically are you gonna play games? if not then mostly a 64mb graphics card will be more than enough spend other money on RAM for your system, 512mb now a days is good, what with the amount of programs you need for everything and how bloated they are.