A Q9550 will be a very good upgrade for $230 roughly. It is the best Q9xxx CPU IMO. I would go with the Samsung Spinpoint F3 500GB hard drive for $50, which leaves you $300 for a video card. What is your resolution you are gaming at? A GTX275 is a good choice or the GTX260 Core 216 might be good enough.
I agree with adding a new HSF, such as the Xigmatek Dark Knight S1283V for another $45.
You're on the right track, but there are some upgrades you could make. The Samsung F1 drives are the older models. Swap that out for a new F3. It's actually cheaper and considerably faster than the F1.
The motherboard you picked runs its PCI-E slots at 4x speed when more than one GPU is installed. You want to be sure to pick a motherboard that runs at 8x speed. Your RAM is also very overpriced. I would get this combo instead: