Yes, and most s939 mobos only support DDR400, but if you put DDR500 in there, it will run just fine. But then you start overclocking, and the RAM doesn't get overclocked until you reach a 250FSB.
You can still get DDR800, even if the mobo supports DDR667. The RAM just won't run full speed until you overclock the CPU.
AMD64 X2 4400+
DFI Lanparty Ultra-D
2GB Kingston Hyper-X (2-3-2-6)
TT Big Typhoon
eVGA 7900GT KO Superclocked (650/1650)
250GB Maxtor DiamondMax SATA2 HDD
Hiper Type-R 580W SLi PSU
19" Sony Trinitron CRT