Another gig of ram really help my setup?

Before scapping everything and building a new system, I was thinking of cheaply upgrading my current set up without buying a new MOBO/CPU. Do you think another another two sticks of 512mb would really help with online gaming? I am concerned with possible dual channel issues with 4 slots filled. It seems my bottleneck is with the mobo because I could only upgrade that to the overly expensive 3.06 533mhz Northy. Just bought the vidcard recently so I'm not not thrilled with getting another one.
Another two sticks of my mem is going for $125.00. For what I can get for my memory, selling my current mem and getting two 1 gig sticks would be more expensive.