I had an X2 5200+ and it quite severely bottlenecked the 8800 GTX I originally had it paired with. When I upgraded to an X2 6400+ it was much better matched and I got a lot better performance in games.
Long story short, I wouldn't pair an X2 5200+ with anything more than a single 9600 GT or HD 4830. Maybe an HD 4770 or 9800 GT.
Sure it could. A 9800 GTX+ is a bit stronger than my 8800 GTX (which again was a little bottlenecked by the X2 5200+). You won't get the card's full potential, but it still would give you good performance. Here's another good match for the price: