I like going half-way.

You can find 2GB (1GBx2) DDR2-667 kits with good timings for a reasonable price.

Grab yourself a E6600 (2.4Ghz, 4MB) and bump her up to 333Mhz - you'll be running faster than an X6800 and saving a heck of a lot of money.

The memory listed here are for those that really want to break some speed records.

it sort of depends.
with the new intel CPU's having very low multipliers, high FSB needs to be fed to get good clocks.
Overclockers buy the cheapest CPU a lot of times (=> a CPU multiplier of 7), and feed it sky high FSB and RAM speeds.
Also, memory bandwidth plays an important role, so 1:1 FSB:DRAM is a good choice. And then on some boards, 1:1 is the lowest ratio available (Gigabyte DS3 - a popular midrange overclocker board)

All these points make having good FAST RAM necessary.
Agreed this RAM is not for all people, but it will suit some hardcore overclockers very well.

normal overclockers would be happy with any memory that can do DDR2-800, implying a 1:1 FSB of 400 MHz (1600QP-FSB) and a CPU clock of 2.8GHz on the E6300/3.2GHz on the E6400