The 9950 is an original Phenom @2.6ghz. It should not bottleneck too badly but there will be a bottleneck unless you super stress the GPU. You should still see quite noticeable gains going to the 660 even if there is a bottleneck.
I would consider either getting a new platform (FX6300, i5 3470) or grabbing a 965BE [$80] if your motherboard can support one.
What resolution do you want to play at and what games?
Usually 720p, sometimes 1080p.
All kinds of games really, but i do tend to be drawn mostly to RTS games.