I actually have a GTS250 SLI setup. Its paired with an older Intel Core2Duo E8400. It runs games great! Everything I've tried to run on it at medium or higher. It should be quicker than an older GTX280.
It would be as fast as a GTX285 or a Radeon HD5850. This is equivalent to the modern day Nvidia GTX750, 650Ti and AMD R7 260X. Keep in mind that the GTS250's wont run DX11.
It would definitely keep up with today's games.