Yes. New ones sell for ~6.5k and the price quoted is decent. But u need a good power supply to run the card. The generic powersupply that comes with the cabinets will not be enough to power the card and trying to use them may even damage the card. What Power supply do u have ?
Edit: Just checked the previous posts. Seems u got a CoolerMaster Silent Pro 500w

You are good to go. I would suggest u to change the processor too. It will act as bottleneck IINM. Try to get a used E7300/E7400(3-3.5k). You should find decent deals in the members market.
Also the HD4850 decent enough for HD gaming but you cant expect it play all games with high settings in HD resolution. As you have a very good PSU try getting a used HD4870. Its better for HD gaming and slightly out performs the HD5770. But change the proccy first.