Searching for a graphics card

well the 4770 is back up to $120 so its still over priced
the 4850 if your monitor is 20 inches or smaller
the 4870 is its bigger
and the 4890 if you just want to max out everything

both the 4850 & 4870 offer the best return on your money

You may want to go for the 4850, I'm a bit worried about you PSU. What make is it? You do know that you could F'up your rig if your PSU goes bonkers.

your PSU can handle the 4870
some people don't care for Rosewill PSU.

the 4870 1GB would be a really good fit for your system

^+1 But look for the 1Gb HD4870, it's larger memory handles AA better.
Also consider the GTX 260 if you prefer Nvidia, the two cards perform much the same and require about the same amount of power.
Best choices : XFX, BFG and EVGA because they give the longest warranties and have the best support.

Price depends on where you shop: Do n't just look at one place, they all offer specials, usually during the weekend, so look especially hard then.
Reread my last post on who is best but remember; that quality of service usually costs a little more, but if you plan on keeping the card for a few years, I would advise you to pay that little bit more and REGISTER the card as soon as you open the box or you'll miss out on that long warranty.
Take a look at the Newegg site and read the reviews there if you find a card you like with good reviews then buy it but do not buy the first one you like the look of and, once you find a card you want, shop around and see if another supplier has it on offer.

Unless it hurts the wallet or the plastic (in my case both), it never hurts to have the extra power. Eventually, all gamers migrate to a 24 inch or larger monitor because it never feels the same when you game on a friend's system with a larger monitor and you come home and find yourself squinting at your 21 inch monitor, lol.