i would suggest you return the 8800gt and get a 4850 instead. they are very cheap(starting at 169 at newegg.com or at best buy for 149), faster than the 8800gt, and uses less power than the 4870. besides you have a crossfire motherboard. i would change out that psu though, to many amps for that psu. besides your only playing at 1400x900. the 4850 is more than enough for that res. so return the 8800gt and get a new 4850 and new psu for the same price as a 4870. http://www.bestbuy.com/site/olspage.jsp?skuId=999998200...
I agree with the HD4850... if it's very little money.
The HD4870 is definitely tooo expensive for the reoslution you're using it at, but if the HD4850 were $150 or less sure.
Over $150 it'd be a tough sell, the GF8800GT is a pretty competent card, and will do pretty good at that resolution. Only if you wanna crank the quality setting or AA would I recommend an upgrade, and really depends on the price, an HD4870 for $180 more definitely not a great deal IMO, especially if you have to monkey with a rebate ontop of that.