yeah, but your psu seems suspect you should look into that, open the case, read the specs, and even at that it may not provide the specs it says if it is a no name psu, not a smart idea to run a *** psu, just saying.... for 1080p I would go no lower than a gtx 560ti/ HD6950, you will also need to oc your cpu a bit.
I agree with you need to check the CPU but you don't need a 560Ti or a 6950 as a minimum for 1080 more like maximum before its overkill. I have 2 4870s in crossfire which gives performance somewhere between a 6850 and a 6870 (no direct X 11 though) and I can play all the games I have on Ultra settings with no lag.
I would also recommend trying to OC your CPU to around 3.4 ghz if possible bc any graphics card that you buy will be a bit held back with your dual core clocked at 2.8ghz, so to obtain optimal performance given your hardware, OC your cpu a bit if you can, that is the only other thing I would recommend to someone in your situation
If you know the exact power supply model we can be of further use to you.
We need too know the amps on the 12volt rail of your psu, not enough amps and you'll underpower your graphics card.
What I have found indicates that you need about 12-15amps on the 12 volt rail to run a 6850 too be on the safe side lets say the 6870 requires 20amps. If your psu matches that you should be good to upgrade to either of those cards.
A quick search said the 9800GT needs 22amps, by that knowledge we can assume your psu has atleast 22amps which should make it able to run a 6850/70 which are the cards most would recommend.
no the 6790 is a trash/fodder card, bad power consumption/performance, and bad price/performance, Do not get a 6790, get either a 5770 or 6850 they both respectively offer lower power consumption and better price/performance