I need help with video cards

I have a Dell XPS Dimension 600 CPU and a 24" monitor and I added a 30" NEC monitor and want to run it in dual monitor mode. The computer has a Dell GeForce 7800 GTX 256MB PCI-E card that only allows up to 1920 x 1200 resolution.

Any ideas on what I need to get the NEC to run at its full 2560 x 1600 resolution? I've gone through enough video specs to make my head swim. I've considered trading up to the 8800 GTX card but have read that it would run too hot in my system.