Cant compare prices for RDRAM and DDR directly. Use the 256 stick of RD as you have to run 16 bit RD in pairs
512MB PC2100=2 256MB RDRAM PC800 pretty well.
If your building a new rig RIGHT NOW iD recommend DCDDR in the GB flavor, coupled with a P4 2.4B, and depending on budget GeForce 4 ATi 4200 or a Radeon 9700 Pro (or not Pro, budget). If you gots money set up a RAID 0 array with 2 Western Dig 800BBs. Sound go for a SB Audgy MP3+ or X-gamer (same hardware, dif software). Case get whatever you think looks good. Monitor grab a 19" from sony, samsung, or viewsonic depending on budget. Optical drives 48-52X CD-RW from Liteon, with a DVD-ROM from lite-on, plextor, etc.
If your going real high budegt configure the same with SCSI and buy your ferrari (sp) too
If building a little ways down the road, same dressings, with a Placer (springdale??) based board (800Mhz FSB) paired with DCDDR PC3200 and a 2.8-3.2Ghz P4C. Throw in the GeForce FX and you have one monster rig on your hands. Huge bandwidth, HT, 800Mhz FSB, and a sweet DX9 vid card
If you dont understand a word of what I just said above, you can safely ignore it