http://www.hardwaresecrets.com/article/26/1
anyways if u want to overclock only ram u should do it by changing fsb:ram divider
either it is ratio and u should go step by step in direction of 0 or by choosing ram frequency and step by step higher and at a given limit when no further step is stable loosening timings
all depends what settings u can change on your motherboards bios
for example i dunno what stuff u have but if u had cpu with fsb 200 mhz and ram currently on 266 mhz u have ratio fsb:ram 3:4 so if your mobo offers u u can change ratio to something smaller like 3:5 1:2 --->0 closing to 0 but u have to do it slowly by small steps and if your motherboard doesnt allow changing ratios u can set 400mhz instead of 266mhz for your ram but i guess if u do that your mobo wont stand up without changing timings but as i said all depends on your bios settings and your ram capabilities