ive ran 8x 1900 QDR which is 3.8ghz on this q6600. Stability started to become an issue at that fSB however. Instead of full stability occt/orthos overnight at 3.825ghz with the 9x multi , the 8x multi and 1900 QDR would only be stable around 1hr in Orthos. SO I'm blaming it on the motherboard.