I need suggestions for a new CPU, MOBO, RAM

I have finally decided to upgrade my computer since its starting to show its age. I need a new CPU, mainboard, and better RAM.

My current PC specs are:
Enermax 535W PSU
Intel P4 2.0 GHz, socket 478
Gigabyte GA-8IEX MOBO
2x512MB Kingston PC2100 DDR SDRAM
6x HDD's, various sizes, all IDE
Promise Ultra TX133 PCI controller card for the HDDs
Soundblaster live 5.1
MSI 128MB 6600GT vid card, AGP
netgear wireless card
Lian Li PC-60 case

I would like to spend about $150-175 each on the processor, mainboard, and RAM (so $450-525 total), and it is VERY important that I get a processor that runs cool, as my computer already runs a bit warm and I don't want to add much more heat to it.

I am thinking of switching to an AMD processor, but I truly am clueless about mobos and CPUs, so I need major help. Is socket 939 any good, and does it run cool? I would also like a MOBO that has at least 3x PCI slots, 2x PCI-E, 1x AGP, and built-in RAID. I have no idea what type of RAM to get either, but I would like 2x512MB peferrably. Anything is better than my PC2100 for now.

Any suggestions on what I should purchase are very welcome. I really have no idea where to start ;_;

Thanks for your help.

>>1
several months ago i put together a dual opteron system for about 500 dollars. that is without video card. you can have a leet system for the money you want to spend.