Actually, the Q9300 quad core processor is very capable. It really depends on what applications you plan to run. If you plan to use the Q9300 for gaming, then it will perform very well. Take a look a these gaming results:
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=3641&p=7
This chart shows the Q6600 and the Q9650. The Q9300 results should be somewhere between these two results. The Q9650 actually outperforms the i920 in Fallout 3, Left 4 Dead, and Crysis Warhead. One would expect the Q9300 to lag behind the i920 in these games, but I suspect that it would have a very respectacle showing.
Most games are GPU (graphics card) limited and not CPU (processor) limited. Pairing a Q9300 with a good graphics card (ATI 5850) would be a very good gaming machine. If this is a used system your looking at, you should give it serious consideration if you can get a very good price.