mid/high end gaming computer

Since you aren't doing SLI and (IMO) DDR3 is much less performance for the price compared to DDR2, I would switch to a much cheaper P35 motherboard and DDR2 memory. That's $300 right there, almost enough to get a nice 24" monitor to go with that GTX280. Or, if you already have a monitor, think about another hard drive for Windows.

You could also probably get away with a PSU between 600 and 700 watts depending on the brand,

That depends on how long you're gonna keep the pc. If say for years, yup go for it. If it's until Nehalem, pick up P35 + DDR2.

X38 is a Crossfire board. You are getting a Nvidia card.
Do you want to switch to AMD?
Or do you want to go the 790i route?

how do you know if its crossfire or not

Please do not get a P35 mobo. Go for an X48 for best performance. It appears as if you are willing to pay top dollar for your parts, so here is what i recommend. Good choices for everything except mobo and cpu cooler.

Here are my recommendations:

Thermaltake Armor+ (it has a removable mobo tray)
http://www.newegg.com/Product/Product.aspx?Item=N82E16811133056

Gigabyte X48 mobo
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128330

G Skill (2x2GB) at tight timings RAM
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231167

CPU Cooler
http://www.newegg.com/Product/Product.aspx?Item=N82E16835202007&Tpk=vendetta%2b2
or
http://www.newegg.com/Product/Product.aspx?Item=N82E16835887011

Thermal Grease Artic Cooling MX2
http://www.newegg.com/Product/Product.aspx?Item=N82E16835186020&Tpk=artic%2bcooling%2bmx

DVD Burner (Samsung is much better and cheaper)
http://www.newegg.com/Product/Product.aspx?Item=N82E16827151154

PSU (modular Corsair and cheaper)
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139007

HDD (Samsung Spinpoint [32 mb cache] and faster than Caviar and quieter)
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152100

I personally have a Q9450 myself but its a pain to overclock. However, the Q9450 is a great cpu. You can easily OC it to 3.2 GHz amd have it run in 1:1 with your RAM at 1600 MHz speeds. However, anything more than 3.5 GHz will require days of work. Most people experience FSB limitations after ~460 FSB. I haven't been able to get past 450. Hope this helps and by the way I doubt you will experience any lag in games since you're gettin a GTX 280.

X38, P35, X48 = Intel; support Crossfire and sinlge nNvidia only

650i, 680i, 780i 790i = nVidia; single ATI and SLI capable

DDR2/P35 = cheap and good

DD3/X48 = expensive and a little better. But a lot more expensive.

btw theres a combo deal on the q9450 with the psu i just posted its a $30 instant rebate

rubix_1011 : iam using MSI 750i, looking forward for great performance / value & good IQ card

I think you're spending too much on the case, probably should look at a Xigmatek s1283 Cooler over the Freezer 7, and would be better served at less cost by two 9800GTX in SLI (but not on that motherboard).