I'll go along with that.
SCSI isn't all that it's cracked up to be anymore as far as speed. It's great for other things (hot swap, large disk arrays, 15 devices per channel, etc.
The 74GB WD Raptor (10k SATA) is simply killer (in price and performance) I have the 36GB model and it's insanely fast. Plus the 15k drive will sound like a dentist drill and not yield any extra performance for your desired goal.
Ram speed won't matter nearly as much if you're not overclocking. Make sure that the motherboard / CPU setup can even take advantage of the extra speedy / extra pricey ram. Just don't go too generic with ram. the cheaper the ram you get the slower the CAS speed (cas2, cas2.5 etc)will be, and performance will sufffer a little.
Lastly, don't buy an Nforce or ATI multimedia chipset board with everything built on. I'd go with something VIA based with built in sound and 1000mb lan and SATA Raid, get a decent pci-e or agp8x video card, and whatever else you want in there and some good ram.
www.newegg.com is a great place for parts