It will be only a little faster, GTS250 is built on the G92 chip the same used in 8800GT/GTS 512/9800GTX/GTX+. 9800GTX/GTX+ have higher clocks for core/mem/shaders and cause that they are only marginally faster than 8800GTS 512MB and 8800GTX (GTX is G80).
G92 is only a die shrink of the G80, from 80nm to 65nm and then to 55nm ( in 9800GTX+ and GTS250 ) and has 16 disabled cores in 8800GT
If you're looking to upgrade from your 8800 to a gts250, it will be worthless. Better spend the money on something like HD5850 or GTX470 (GTX470 still isn't available, but you can preorder)