I'd recommend starting out with explaining what you want to do with the system. Explain realistic expectations you want to get out of it. I'm having a hard time imagining a preference for 1GB of GDDR3 over GDDR5, other than trying to create a break in price points that isn't really there. Is there reason for the 1GB size?
What monitor are you using? What resolution? What are your other system specifications? What type of processor? Every choice dictates the options available for the next choice, and so on...
The market's really fragmented. There's a ton of processor choices that will dictate what motherboard type you choose. The motherboard choice can effect what type of RAM and how much you can put in it. It'll also determine what is xfire ready. All of that combined will impact the choice of power supply, case, cooling options, etc.