Like everyone else, the 5870 was a monster in it's day. You can mostly point to the "breakthrough" consoles for pushing the boundaries here though. To this day, it's still very relevant and quite capable.
"The reason I ask is that a friend of mine has two cards a GTX 550 Ti and a HD 5870. He will sell me either one for about the same price."
The 5870 will use quite a bit more power, but it should just walk away in many games.