What should I upgrade?

[Coburn]

I want to overhaul my computer, but I can't decide what components to do first. Here's my complete specs:

Case that supports only MicroATX
PSU: 300 watt
Mobo Intel Desktop Board D945GCL
CPU: Pentium 4 3.0 Ghz Hyperthreaded
CPU heatsink: CRAPPY
RAM: 2GB PC4200 533 Mhz
HDD 1: SATA II 160GB
HDD 2: PATA 200GB
Video Card: nVidia GeForce 8500 GT-1GB DDR2 VRAM
Sound: Onboard

I want to upgrade my computer so that each new component will comply with my old stuff. I think I am going to start with the case and power supply. What should come next? A 500 GB hard drive, a new motherboard, or a new CPU (my mobo can only support core 2 duo and I want the quad Q6600)? What wold be most beneficial to me?

[Boost]

if you're getting a case and PSU I'd look into just getting a barebone kit that supports the Q6600
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=4282930&Sku=P450-9112 C
would be a nice upgrade kit

[Coburn]

Nice kit, but the mobo only has 2 RAM slots. I am gunna get 4. But I will look into barebones when I have the money (summer job LOL)

[Snafupossum]

if your motherboard supports 45nm cpus, i would get a core2duo 8500, it has the same performance as the c2q 6600 in games, its only in number crunching apps that can actually use the 2 extra cores efficiently like a nice video trans coder do you see the performance increase in the quad core. but i have looked at countless benchmarks on many sits and they are say about the same thing. I was going ot upgrade to the 8500 myself, but I figure my computer is good enough as it is now.

[Coburn]

if your motherboard supports 45nm cpus, i would get a core2duo 8500, it has the same performance as the c2q 6600 in games, its only in number crunching apps that can actually use the 2 extra cores efficiently like a nice video trans coder do you see the performance increase in the quad core. but i have looked at countless benchmarks on many sits and they are say about the same thing. I was going ot upgrade to the 8500 myself, but I figure my computer is good enough as it is now.

Well, I'd be using the CPU for archiving, video encoding music encoding, picture editing, gaming, dual monitors, and general multi-tasking. So would a duo be more efficient for me?

Also, what does hyper-threading do for my CPU? I'm running one now, would I see a difference if I got a duo or quad? Also, how can I check what nm of processors my motherboard supports? It just tells me core 2 duo is supported. Here's my motherboard:
http://www.intel.com/products/motherboard/D945GCL/index.htm