Well depending on what your other parts are, interested in the PSU, it might be okay. I'd consider getting the q6600 over the q9400. The q6600 can easily be OC'd to 2.8-3.2 gHz on stock cooling and it costs about $70 less. You can use the $70 less for other parts or put some of the $ towards a better CPU HSF. It's up to you, but that is one thing I'd consider. You should also consider getting DDR2 @ 800 mHz (4-4-4-12 timings) on stock voltage settings (1.8v). This will allow you more OC'ing room (if you decide to do this) and will also run a bit cooler, due to the lower voltage. Your mobo is a smart choice and should be a good one too!
not really looking to overclock, just want something good and powerful enough out of the box. Want something that will last another year or two, but do not want to spend the loads of cash on the latest and greatest (ie: i7 ddr3 etc)
Almost the same exact setup I have at home!! LOL!! I don't have the 8800gt (7900gs ). A couple of DDR2 sets to consider for your build!
The q9400 should do fine too, just thought it might be cheaper to get the q6600 and just OC it a bit to get more performance out of it for no more $!! You could even get a nice C2D for under $200 that would do just fine too. It all depends on your applications and uses.
Too bad socket 939 was so short lived and that you didn't get an AM2 board instead. The build looks good. You could save some money by going with DDR2 800, as 4GB of the stuff is so cheap now, but if you can afford it that's a good RAM kit. I'll probably be getting 4GB of Mushkin 1066 myself soon.
Just keep in mind that you don't have as big an upgrade path as far as the CPU is concerned since socket 775 is at the end of road it seems as Intel is replacing it with its new platform. If you are concerned about upgradability you may want to consider an AMD Dragon platform as one of the things it has going for it is that most boards that qualify to be part of the Dragon platform should support future AM3 CPUs with just a simple BIOS update.
Even so considering that your motherboard can support up to 16GB of RAM and that you can overclock that Q9400 in the future when you find it lacking, you should be fine for at least another 3 years. I'm sure you will notice the increase over your current computer immediately. As lunyone suggested the Q6600 is a pretty attractive option for it's price, but clock for clock the newer 45nm Wolfdale CPUs like the Q9400 are faster and have more overclocking potential.