If you are speaking about windows XP you couldn't be more wrong. Xp only supports 4gb of ram, and it only registers 3.7~. If you are on a 64-bit system you can use as much ram as your motherboard can handle.
Upgrade your ram before alloting more memory for video. You don't have enough ram for it to be worth anything. I only have 2 gigs on my desktop. DDR2800. I am going to add either 2 more gigs or 4 more. I am still unsure.
its really pointless to have more then 4 gigs unless you have a 64 bit OS. and even then you dont even really need that much ram. i only have 2 gigs of ram in my comp with a core 2 duo processor and it flys. i love everything about computers its like my favorite pass time. LOL