OP just as a suggestion I would not purchase a new quad solely on the expectation that you will overclock to get your desired performance increase.
A Q-9300 is at 2.5Ghz with 6Mb of cache. I don't know where you are purchasing your chip but amazon sells it for 208.00. These quads thrive on high cache amounts. 6Mb is ok. But if you are serious about upgrading just your CPU for a good performance gain you should really look at a Q9550. This chip has 12Mb of cache and is stock at 2.8Ghz. This chip will start you out with better performance and be easier to overclock with a non-overclock friendly board.
In all honesty if your spending this much money on a CPU you should opt to spend a small amount more and get the better chip. If you get the 9300 and want to overclock you also have to factor in the Heat Sink and Fan, the stock is not good enough to do any kind of large overclock 2.5 -> 3.6. So this is an additional cost. $30.0 for a decent new HSF.
CPU Comparison - you can play with the toolset at the top to see other tests.
Another thing to note is that the Q9300 has a small multiplier of 7 which means your Mobo will need a high stable Front Side Bus. This means it is a harder chip to overclock. Your mobo is extremely limited to what you can change so it might not work or even work to what you expect.
If you are going for this upgrade I'd really consider you look at the Q9550 over the Q9300. I think you will find the Q9550 will give you great performance even at stock.Edited by Aparition - 3/11/11 at 6:23am