You've got your CPU and graphics back to front! The higher the res the less the CPU comes into play and the greater the burden on the graphics card.
If your 9800GT is a 1gb version then you could by another and SLi them if you buy a SLi capable baord, this would do pretty well in 1080p, but only really worth it if they are 1Gb versions.
Otherwise as Wa1 says you want an ATi 5850/5870, or perhaps a Nvidia GTX470. I think the Nvidia cards are a bit too hot and power hungry this time round and the GTX480 is way too expensive. 5850 is probably good enough for most things in 1080p, a 5870 would be ample for everything but Metro 2033 in full but even a 5770 is still fairly good for 1080 too and would be the only other budget option. Make sure whatever you get it has 1gb+ of RAM.