Ok basically your CPU speed = FSB x CPU multiplier.
Your RAM runs at the speed of your FSB. So DDR 400 which runs at 200 mhz is what most people by because that is the speed of the current AMD CPU's FSB. Now an Opteron 165 for instance has a multiplier of 9 so 200mhz (fsb speed) x 9 (cpu multiplier) = 1800 mhz (cpu speed). Since multipliers are usually locked on most processors people must overclock by raising the FSB.
So if you went and bought DDR 500 which runs at 250 mhz you could raise your FSB speed to 250 mhz while having your memory run at the same speed, so this is what you could do. 250mhz (fsb speed) x 9( multiplier) = 2250 mhz.
You will also be able to raise your RAM speed but there will be a certain point where it becomes unstable. the DDR 500 RAM would be able to reach a higher speed which would enable you to get a better OC without using a memory divider. Memory dividers run your RAM at a fraction of the speed of your FSB which makes it slower.
So basically people buy faster RAM to OC.
AMD Athlon 64 3500+ @2.6 ghz
Arctic Cooling Freezer 64 Pro
2gb Corsair XMS DDR400 RAM
ATI Radeon x1950xt
MSI RD480 Neo2 Crossfire Mobo
Western Digital 160gb SATA HDD
Fortron 450 watt PSU
KINGWIN SK-523BKP Case
Logitech MX518 Gaming Mouse