Guys, stop giving bad advice. There needs to be some serious cleaning up on these forums.
First- There's maybe... 1% of the current programs out there supporting quad core architecture. You're lucky to find programs that support dual core. If you plan on over-clocking, your money will be better spent on better cooling, because the E6600's run REALLY hot over-clocked. E6420's and E6320's actually run about the same as E6600's in newer weeks. A quad core at this point will be wasted if you don't game and play newer games. Unless you do some very extreme auto-cading and encoding and rendering, you won't see a benefit in quad versus dual.
If anything, a dual can clock higher than a quad and would give the dual a benefit. Check Tomshardware or Anandtech, whichever reviewed the QX6800. It was basically neck and neck with the X6800 with only a FEW programs showing improvement with quad. With rendering, encoding and various other work-station tasks, the X6800 was virtually the same.
Second- Unless you jump into an 8800GTS/GTX, you won't really see any rendering or workstation performance jumps that are worth noting, but the 8500GT is a crummy card regardless, especially in a "fairly expensive" build. I recommend at the very least, an 8600GTS with a strong recommendation of getting an 8800GTS 320mb version. You'll see a huge improvement and your system will have some future proofing to it. No need for 640mb, as a casual gamer has no need for massive frame buffer. VRAM in rendering isn't as crucial as the core's power, and while there's some benefit to having more VRAM with rendering, the gain doesn't outweigh the cost.
Third- That virtual hard drive thing is basically a gimmick, not worth getting. You'll get surreal fast loading times and access times with a simple Raptor, and you don't need anything faster than a 74GB Raptor for your programs. Seriously, when have you ever had more than 40-50GB of software? I'm a writer, do some encoding and video-editing and a heavy gamer, and the MOST I've seen on my software drive was 45GB of programs. (3d Studio Max, Maya, Adobe Premier, AutoCAD, Office, Open Office, Fruity Loops, VEGAS, Acid Pro, a bunch of games) I think you'd see more out of getting good ram, a good raptor drive, and using the rest of your saved funds on your video-card improvement.
This is my short list for you.
*Intel Core 2 Duo E6420 2.13GHZ $186.00
*G.Skill DDR2 800 2GB HZ kit $109.99
*ASUS P5K Deluxe $224.99
*Western Digital Raptor X 74GB 10,000RPM $154.99
*Western Digital Caviar SE16 250GB $69.99
*EVGA 8800GTS 320mb $279.99
*Silverstone OP650 Olympia 650 watt power supply $139.99
At that point, you can pick your case, software, etc. With this set up, you get the benefit of a strong running system that can overclock really well, can give you the performance you want in every aspect and has some future proof to it.
I recommend picking up a good cooler if you want to over-clock, or you might even want to consider going water-cooling to guarantee you get the performance you want with the security of good cooling to keep it stable.