Any of the 5770s should fit, provided you don't have something in the top PCI-e x1 slot. Also, out of curiosity, how do you have 3.5GB of RAM?? (From your signature)
I'm supposing OP has 4GB of RAM but is only using Windows 32-bit. Then again, apparently with 1GB video cards, you'll only have 2GB or 3GB of RAM. OP could be running three sticks of 1GB and one stick of 512MB.
You'll need both the same size and the same frequency/latency. Dual channel doubles the available bandwidth between the RAM and the memory controller which can help prevent some bottlenecking.
oh, damn well not much i can do there, unless i buy a new 2gb stick an take out the 1gb and the 512mb. but as you can see i think my current piece of junk video card is the biggest bottneck. thanx for the help though. eventually maybe ill just get another 2gb stick and make it 4gb