First, the CPU you picked (Q9550) is an awesome choice. 12MG L2Cache is much better than an E8600 for the same money. The new CPU price War just came to my attention last night.
Just be sure to OC it.
Second, if you can find a Mainboard : Gigabyte P35-S3G (Chipset : Intel P35) like mine for $79.99 get it. It is so dependable that all the people I told to buy it are nubs but none of them have needed one single second of help.
Third, the Video Card choice must include a Trade Up strategy approach. In just the nick of time I traded up from a GTX280 to a GTX295. And did it early enough to avoid the frenzy.
Forth, the frenzy. The GTX295 is the only card to consider. But it's out of your price range. Every body has learned that the GTX295 does not have the Micro Stutter and SLI Profile problems like the 7950GX2, 9800GX2, and even the 4870X2. Just try and get a GTX295 at Newegg. http://www.newegg.com/Product/Produc...d&Order=PRICED
So, upgrade your existing computer, use the 7800 you already have and watch the prices carefully. Most importantly figure out a Trade Up strategy. 90 to 100 days after purchasing a GTX280 for $450 I almost Traded Up to a GTX285 that retailed for only $379. And I would have had to pay nothing. But luckily I spent a mere $99 more for a GTX295 Trade Up. If if I call the next nVidia series the GTX300- idk what to call it- you have to strategize (Google) when will it be out.
The performance going from a 7800 to any GTX200 will blow you away- along with a Quad CPU with 12MB Cache. I had a 7800GT on a 939 system and just going to an 8800GTX was SICK! But what if you got a GTX200 series at just the right time so that it was possible the Trade up for a GTX300 series?
Besides, see what improvement the new build with the same video card gives you. By the time you purchaise, build, tweek, tweek, so much time can transpire. And before a new card the system it goes on should be super duper Road Tested. While you shop, wait, build etc. you can research.