Doomworld has a good collection of much of the Doom 3 info that has leaked out so far.
For the highest-quality cases and cooling equipment, hit the Cooler Master site.
AOpen offers high-quality computer cases at excellent prices.
ABIT is the home of the attractive and responsive IC7-Max3, one of the best mainboards on the market.
The highest-quality performance RAM is by Mushkin. Well worth the exorbitant price.
Tom's Hardware Guide is a useful resource for the latest news and reviews for top-of-the-line hardware.
Western Digital's Raptor high-speed serial ATA drive competes with SCSI in speed, but without the SCSI hassle.
SiSoftware Sandra is considered the gold standard of software benchmarking tools for your system.