If you need to change that many things, your better of getting a new computer...You can build a mid-high range computer yourself for $700-$900 depending on what you get. Or you could build a new one and reuse some parts, but that usually not a good idea.

Okay, first off, If you're on budget, take the 9600GT, its less then a(Cant remember, 30 comes to mind, but I think its alot less)30% difference in performance, and if overclocked is within 5 FPS of the 8800GT. Its also $40 cheper, though 8800GT prices are slowly dropping.

On the CPU, you have to ask, "what am I using the computer for?" For gaming, and most people, a Quad-core is unnecessary right now. Not to mention, a new batch of them(highly expensive ones), just got released, and the Q6600 is at the bottom of the spectrum, and in time, will be lagging behind newer CPU's. If you plan on having a few browsers open, downloading stuff(Pr0n, music, pirated games, your call), using it for art and design, etc. Then a quad core might be considerable. I'd keep on Duel core until its nearly standard for games to support them and you get a real boost from them.

The E8400 is WAY better then the Q6600 for games. It has higher core speed, higher FSB, and better at overclocking if you want to do that, plus it about $30 cheaper.(Even after recent Intel Quad-C price drops)

You should get a new PSU, too. Definitely. If your computer is old, then your PSU is reaching the end of its lifespan anyway...400W probably won't be enough for a new GPU and CPU. You shouldn't need much more though. 550 or 500 would work, but personally, I like to keep it at 600W- not to much, not to little.

Motherboard, it really depends on what you want to do. I personally don't know much in the way of motherboards, but there re high-end ones, low-end ones, I really need to know exactly what you're looking for.

Memory is easy. G-Skill, Corsair, Patriot, OCZ, and Kingston are all reliable brands. G-Skill has great value, and Kingston is mostly cheap quality memory. Corsair, OCZ, and Patriot  have alot of "high performance gaming memory", if you want to put in a few extra bucks for EPP, nice heatsinks, and lower latencys. Usually any will do.

HDD, just any Western Digital or Seagate, you can get a higher speed(10,000RPM)160GB, or regular but with tons of space. If you're on a budget, or dont need alot of space, just get a nice 250 or 320GB(Preferably 320, theres only a $5-10 difference)

Disc drive, any will usually do, whether you want DVD, CD, DVD or CD BURNER, Blu-ray, etc.(Though you probably shouldn't get just a CD drive).

Oh, and stay away from AMD and ATi stuff, Nvidia/Intel are WAY better.

And buy stuff off Newegg.com or at least use it as a guide.