System RAM and Video RAM (VRAM) work independently of each other.
Since you posted this question in the Graphics Card section I assume you are asking if you can install a video card that uses DDR3 or DDR5 VRAM in your computer that probably only uses DDR2 RAM.
The answer is yes. My Abit IP35Pro motherboard only accepts DDR2 System RAM, but I have a HD 5850 video card installed which uses DDR5 VRAM.