The 8600 GTS will be noticeably faster than the HD 2600 Pro.
To be honest though, both of these cards are extremely dated and underpowered by current standards and I would not recommend spending anything to obtain either.
What ever budget you have, no matter how tight, should be saved for a much more modern card.
its marginally worse. but in reality both are to weak for todays games. both register at around 250 on pass mark, while a 9800gt which is minimum recommended spec for new games pushes 700+
the 650ti recomended as the minimum you actually want to buy if your a gamer today, pushes 2700+.
the gtx 780 rocks near enough 8000 points.
i just gave these as an example of how far back the 2600pro and 8600 are and neither are worth your money.