ATI HD4850 is part of the Radeon HD 4k graphic cards family and it features the RV770 Pro GPU which is a 965 milions transistors  chip build on 55nm technology. The default clock speeds are 625 MHz for the GPU and 1,986 MHz for the memory (DDR3).

GeForce GTS 250 is actually a rebranded 9800 GTX which was an overclocked 8800 GTS.  So it features a slightly revisioned version of the G92 GPU present in the 8800 series.  The new G92b   is a 754 milions transistors  chip build on 55nm technology. The default clock speeds are 738 MHz on the GPU, 1,836 MHz on the shaders, and 2,200 MHz on the memory  (DDR3). However it is now a more  mature chip now and the power consumption and temperature have improved compared with the 9800 GTX.

While the GTS 250 scores better on Synthetic benchmarks, games performance results  are split as the HD4850 seems to perform better on some games. With standard coolers the GTS 250 seems to run cooler but this might change if you go for an HIS ATI HD4850 with ICEQ4.