Your CPU is the bottle neck. As you bump up the color and resolution, it puts more load on the graphics card. This is where the TNT2 will show itself on your system. I ran into the same wall with my Voodoo3 2000 when I overclocked it. I got not increase at the resolutions that I was using, but now I can run at the next higher resolutions without any performance hit.
AMD Phenom II x4 945 3Ghz | ASUS M4A77TD | 2X WD 1TB SATA 2 hard drive | 2x2GB Corsair XMS3 | nVidia GeForce 8800 GTS | ATI TV Wonder Theater Pro 550 | Antec P-160 case | Antec 650w Earth Watts | LG Blu-ray Super Drive | LG DVD RW | Windows 7 Pro