i would go with a P4 1.6a chip if i were to look for the best value for money. These chips can easily do 2128 Mhz (2400 Mhz in my case) at default voltage. Some ppl have made them run at 2.6 Ghz too. but thats only if you want to overclock.
If you dont want to overclock, 2.0a seems the best bang.
DDR Setup:
Regarding the motherboard, i went with the giga-byte GA-8IRXP board since it has all features one would ever want (excluding IEEE). Asus P4B266 is also a nice choice. i would choose between these two. Also wait for the NEW Intel 845E chipset boards (533 Mhz). Here Asus P4b533-c, Abit IT7, Gigabyte GA-8IEXP comes to mind. do some research and select the features that you want on yours. i would highly recommend waiting for atleast 1 more week so that all the new boards come to the market.
Go with good quality Ram........Crucial PC2100 or Samsung PC2100 seems fine (Samsung PC2100 @ 300 Mhz here, no problems). however, i would pick up PC2700 just to be on the safe side if you were overclocking but if your not overclocking just stick in a PC2100.
RDRAM:
Again, Asus P4T or Abit THII7 comes to mind. These are the older i850 chipset. wait for the newer boards (533 mhz w/ 850E chipset). Gigabyte GA-8IHXP, Asus P4T533 comes to mind again.
Again go with name brand memory..........Samsung or Corsair seems fine. If you want to shell out the big $$$$$, you might want to go PC1066 but PC800 will do fine. (Note that for Rambus memory, you will need atleast 2 chips to work, so plan accordingly)
Again, it depends on what you want and how much money you want to shell out. You can go I845E-DDR setup or I850E-RDRam setup. The rambus setup will outperform the DDR setup at any time though. I am again highly recommending you to wait atleast 1 more week for the new I845E and I850E chipset boards to come out in the market and then decide.
hope that helps
Thanks