Does installing 2-1gb of RAMs to the remainng slots where two were installed with 512mb, giving 3gb the best option to increas

If you have 4 slots and a dual-channel motherboard, install the 1 gig sticks in the same channel and the 512mb sticks in the same channel. The slots may be color coded, but you may need to check the manual to see which slots pair up. They are not always side by side. Get the fastest RAM your motherboard supports for best performance.