8Gb of DDR2 isn't that bad. It's getting cheaper and cheaper, I bought 8 Gb DDR2-667 (Buffalo) for € 440 (around $600) including 19% VAT a couple of weeks ago.

The nForce680 is nice, but somewhat overpriced. The nForce650 series is just as capable, and quite a bit cheaper.

My own mainboard is an MSI 965 Neo 2, based on an Intel 965 chipset. 5 SATA II ports, 1 IDE port (for 2 IDE devices), 8.1 audio, Crossfire (ATI version of SLI)

The 8600 is a decent graphics card, but not as fast as a 7900. If you want a faster card, you'll need an 8800 - pricey!
The new ATI R600 based graphics cards might also be an option. Somewhere between the 8600 and 8800 in both performance and price, and reputedly good OpenGL 2 support.

By the way, you don't need a sound card. All modern mainboards come with integrated sound, 5.1 or better. A high end audio card is only useful when you're using (semi)professional audio applications. 

Hard drive: don't go for parallel. Go for SATA II. Faster and cheaper. A 320 Gb Seagate Barracuda 7200.10 sells for € 75 including 19% VAT in the Netherlands.

If you can afford it, I'd recommend a Q6600 CPU instead of an E6400. A Q6600 is effectively the same as two E6400s in one package.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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