The GTS 250 is another re-brand of the 8800GTS 512Mb/9800GTX(+) and can use both the G92a (65nm) and G92b (55nm) chips.
As the G92b variant requires less power, these cards only require 1 PCIe 6 pin connector where as the G92a based cards requires two.
Performance is identical between the two although the G92b based cards may overclock further.