I'm no expert. I'm a newbie too.
You have a motherboard that will support dual channel ram. It's got 4 slots for ram.
Buy another 512mb stick of the DDR400. Put 1 stick in channel 1 and put the other stick in channel 3. You'll have 1gb running dual channel.
I don't think your 256mb stick and your 512mb stick can run dual channel, because they aren't the same size.
You'd see an improvement in your machine simply doing this. How much of an improvment? I don't know. Someone else can help you with that one.
I'm using a P4 3.0ghz Prescott 468 pin CPU on a gigabyte board. I have 4 sticks of ram 512mb each. Plugged into 4 channels on the board. My rig will play any game you can throw at it, but my GeForce MX4000 video card sucks. Made a mistake buying that. I'm after a 7800GS now. Waiting on tax refund.
Once I put that video card in--it'll be smokin'. At leat to me it will be. Others may call it a POS, but compared to the PIII 550mhz on a 100mhz board I had--this thing is a rocketship.
I think you got a decent rig. Not top of the line--but it's functional, and fast enough to most anything you need. Fix that ram though. Get 2 sticks, same size and run them dual channel. It can't hurt. Cost ya $50--$60.