New 5850 doesn't perform much faster than 3850, what gives?

[NoQuarter]

bingo. The game is ancient.

If you want better performance with WOW Nvidia cards get faster performance.

It's actually performs equally well on nVidia or ATI hardware, but anything 4850 or GTS250 or higher is going to be plenty FPS on the GPU side, if you're still being held back at that point it's gotta be the CPU especially at 1280x1024:
http://www.anandtech.com/video/showdoc.aspx?i=3650&p=7