They are essentially equal except for a few things. Namely the newer tech that comes with the 600 series card versus what the 6000 series cards were privy to at the time. However in terms of performance they should perform just the same. And I had a hard time finding one on newegg which leads me to believe that at the present time they are both a horrible value card. The only reason I would put either card in my system was to turn the monitor on to view youtube videos, and play facebook games. Any modern game will struggle with either card.
Well even a 440 god thats awful. If you want to play modern games and don't want to spend a ton a 650Ti or 7770 would be worthwhile options on a tight tight budget the lower end cards like a 630, and 640 are for those who run a HTPC and have no desire to game.