If money is no object, then look at the following chart for the HD 5970's performance @ 1920 x 1200 both stock speed and overclocked speed. Playing on a 1920 x 1080 HDTV / monitor will probably give you around 5% more FPS.
The testbed for those benchmarks are as follows:
1. Intel Core i7-975 Extreme Edition processor (3.33 GHz, 6.4 GT/s QPI);
2. Gigabyte GA-EX58-Extreme mainboard (Intel X58 Express chipset);
3. Corsair XMS3-12800C9 (3 x 2 GB, 1333 MHz, 9-9-9-24, 2T);
4. Samsung Spinpoint F1 HDD (1 TB, 32 MB buffer, SATAII);
5. Enermax Galaxy DXX EGX1000EWL 1000 W power supply;
6. Microsoft Windows 7 Ultimate 64 bit;
The i7-975 can be bought for around $950 - $1,000. You can buy a slower i7 CPU and overclock it to get similar results.
Here's the review:http://www.xbitlabs.com/articles/video/display/radeon-hd5970.html