First, there are 2 versions of te cpu. 2.0ghz with 1mb L1 cache and 2.2ghz with 512kb L1.
The first one is better. in your system the memory is big blocker becouse it's allready running at full speed (3200=ddr400). For HOW TO follow the link:
http://www.pcstats.com/articleview.cfm?articleID=1804
i don't recomend any voltage inc for beginers, this is the best way to kill you hardware.
the best way to do it is in bios. stick with the cpu fsb, and try changing the fsb:ddr ratio (if available) to lower the ram speed. you should be fine around 210-220mhz fsb. post your results.
P.S. Read the warnings carefully.