Core2 quad 9550 vs core2duo e8500

They would both perform very well, but if possible I would recommend going for the Q9550. Most of the games coming out in the future are going to be optimized for quad cores so it would be much more future proof. If your building a new pc though I would go for the i5 750, but if your just upgrading the Q9550 is a great choice.

+1 for the Q9550

Quad's of comparable clock speed perform just as well as the dual core equivalent, however some more recent games (not many) are able to use the extra cores.

FSX for example works much better on a quad

Another vote for the quad.