Which video/graphic card should I buy for my gaming desktop PC?

http://www.videocardbenchmark.net/ is the website I use to determine performance. I then figure out how much I want to spend. the 7870 is not bad and should do OK. The critical part is the POWER SUPPLY...you need the PS to have the connectors and wattage to handle the video card in addition to all the other components. There are plenty of Youtube videos to use a guide. Watch them FIRST.

you dont need processor

so will put the money on power supply and graphic card

buy a titan black and call it george...........

First, you probably didn't need to order that new processor. I have the exact same one as your old one and it handles anything I want it to play quite nicely.

I would like to stress that buying used computer parts is risky. Cards can come DOA from the manufacturer due to poor handling of the package. A private seller is less likely to go through their due diligence when packaging the card so it'll be even more vulnerable to breaking in transit.

If you insist on buying used, you'd be crazy not to get that 7970 for 170 on the ebay page. That card commonly sells for 400+. That price is an absolute steal... if it works.

If you want to buy new, the step down from the 7870, the 7770 would work. It's not the best card, but it will play most modern games quite handily and costs about 80 new.

As for installation, graphics cards are piss easy. It's flipping a little lever on the end of the PCI express slot, pushing it in and connecting the power. CPUs are a bit more difficult and power supplies tend to turn into a nightmare of cables and wires. Short of changing out a motherboard, it's the largest pain in the ***.

LinusTechTips is channel on youtube with some build guides. You can also get help at the LinusTechTips forum.