DDR memory was easy to follow. My Athlon XP 3200 had a 200 MHz system clock and a 400 MHz FSB. So I used DDR 400 PC3200 ram, which was 200 MHz clocked and matched with 400 MHz FSB CPUs. Now I have an Athlon 64 X2 4200+ with DDR2 800 memory. But I don't get why I had to buy DDR2 800 memory. Everywhere I read suggested that if you buy a dual core Athlon 64 you NEED DDR2 800 memory, but why?? Isn't DDR2 800 actually 400 MHz system clocked? My Athlon 64 X2 has a system clock of 200 MHz, just the same as my old Athlon XP. So why the need for DDR2 800? Why shouldn't I have bought DDR2 400, which is 200 MHz and would have matched my CPU system clock? I've read a dozen guides on the Internet about it but none of them explain what I'm looking for. Thanks in advance!!