S478 can be 400 (100MHz), 533 (133MHz), or 800 (200MHz). The 1:1 rule applies to bandwidth.
Bandwidth = "effective" FSB x 8
At 800FSB, the bandwidth is 6400MB/sec (800 x 8), so to have a 1:1 bandwidth ratio with the RAM, you should have 2 sticks of PC3200 running in dual channel mode @ 200MHz.
Remember that DDR-SDRAM is named for the amount of bandwidth (aka throughput it can handle), so PC3200 can handle 3200MB/sec...in dual channel mode (x 2) you have 6400MB/sec
ASUS A7N8X-X
Athlon XP 1800+
8.5 x 200MHz
1024MB PC3200 2.5-3-3-7
Asus A9550GE/TD 128MB
WinME/WinXP Pro SP1