Video Memory speeds (DDR2, DDR3, GDDR5) have absolutely no direct affect on your RAM in your system. However, if you're using an older system that utilizes DDR2 memory, that may signal to use you may want/need more system upgrades to gain the performance you're looking for.
All depends on what you're doing.
1) What video card are you considering?
2) What games do you play?
3) What resolution is your monitor?
4) What CPU do you have, and what is the clock speed?
5) Do you have a specific budget for buying this video card?
All this info will help us give you some advice to make sure you get the right video card to begin with.