I'm in the same situation of needing to replace an old Althon PC and have many of the same requirements and questions as bardel.
My current choices are:
Mobo - Gigabyte GA-N680SLI-DQ6 (should be out this month)
CPU - Intel Core2 Duo E6600
Memory - Corsair Dominator Twin 2x2048-8500C5D (2GB)
Hard Drive - Probably just a single Wester Digital Cavier 400Gb or 500Gb (not bothered about RAID)
GPU - 8800GTX (Don't know which yet)
Sound - Creative X-Fi Xtreme Gamer Fatal1ty Professional 7.1
Case - Antec P180 Black
PSU - Corsair HX 620W
CPU Fan - Tuniq Tower
Keyboard/Mouse - Logitech Cordless Desktop MX 3200 Laser
Monitor - Dell 2407WFP 24" Widescreen
DVD-RW - NEC AD7170S SATA
OS - Vista Ultimate (34 bit)
I'm also not sure whether to save money on the RAM by going for 6400 stuff, but the idea of the Enhanced Performance Profiles (EPP) on the 680i chipset appeals.
Any comments gratefully received