Building Semi-New Computer, need advice

[TeeklinDR]

Hey all, my motherboard just kinda died recently so I figured now would be a good time for an upgrade. I've been upgrading this eMachines for the past 5 years and there are still plenty of parts that are usable, but at the same time I need some new stuff as well.

I found a nice Compaq and HP at best buy that I could use the mobo/processor from and put my own stuff in and make a decent computer, but I figured that I could probably just do it myself buying the parts and get it for a lot cheaper than 550.

So let me tell you what I have and what I'm looking for.

I'm looking to use from my old computer:

550 watt power supply
IDE 120 GB hard drive
DVD writer drive
DVD reader drive
2x512MB DDR PC3200 RAM
Audigy Sound Card PCI
AGP nVidia 128MB video card
Wireless Network Card PCI

So what I need to get for a full computer past that is:

Motherboard
Processor
Another HD (internal or external, depending on price)
New case (as big as I can get it)

Now the problem is that I've never ever been big on motherboards and processors and I don't know what kind I need and what will work with what I've got. I like AMD at this point, but from what it looks like if I get an Athlon X2 Dual-Core I can't get a motherboard that uses DDR standard RAM (I'm not sure if that's the case or not), so I'd need to buy DDR2 RAM and that would jack the price up for the whole she-bang a lot.

So is there any single-core AMD processor (looks like 3800+ is what I'm looking for I think) and motherboard combo that will allow me to use my current RAM and all the rest of my gear? Looks like I'll need a motherboard with at least one free IDE slot to hook my current HD up to, and at least one SATA to hook my new HD up to if I get an internal. I'll also need 2-3 PCI slots and 1 AGP slot.

Also, will I be able to use a full tower or is Mid-ATX as big as I can go with what I have? I hear that there is a lot of things you have to do differently when building a full tower computer and I've never built one. If so, what are some good full tower cases that anyone recommends?

And lastly, I'll need a new HD. Should I go internal or external? Needs to be at least 200 GB...any recommendations?

I know my problem is kinda complex and it isn't helped by me having only marginal computer building knowledge, so any help would be greatly appreciated! Thanks a lot!

[FearFactory]

Motherboard Epox EP-9NDA3I Nforce 3 250, DDR400, AGP8x (need bios 05.09.08 to support AMD64 X2 cpu's)

http://www.newegg.com/Product/Produc...82E16813123263

http://www.epox.com.tw/eng/products_content.php?ps=351

AMD64 X2 3800+

http://www.newegg.com/Product/Produc...ubCategory=343

Seagate Barracuda 7200.8 200GB SATA 150

http://www.newegg.com/Product/Produc...82E16822148066

Western Digital Caviar SE16 WD3200KS 320GB 7200 RPM 16MB Cache SATA 3.0Gb/s

http://www.newegg.com/Product/Produc...82E16822136003