GA-K8VT800 PRO Memory Problems

Have a GA-K8VT800 motherboard, I've been having memory problems with
the board since I got it I had 2 512 meg DDR 400 from a previous system that
worked perfect before putting in this system whilst messing around setting
bios I noticed memory speed was on auto defaulting to 166 MHz so I set it
manually to 200 MHz as it should be and my system froze and refuses to
reboot I have to clear the cmos and it then reboots defaulting to 166 again
I returned the memory to suppliers who happily changed it despite it testing
perfectly due to possible compatibility problems again the same problem we
have tried 3 other makes of memory as matched pairs to no avail the system
runs perfectly except that it wont run at 200 MHz memory set manually and if
on auto I assume set by the spd chip it only runs at 166 mhz despite the
same memory running in another system at 200+MHz the cmos doesn't have a
cmos clear jumper so I remove the battery and mains lead for upwards of an
hour sometimes and this still does not always clear the cmos ....any help or
suggestions please or other things I should do such as manually set the
timings as well etc if so whats the suggested settings for good quality
generic ram as opposed to corsair or mushkin