Depends which X4 you have. If its one of the BE chips, just turn the multi up and run Prime 95 to be sure its stable.
As far as coolers go, it depends how far you want to OC. Anything up to about 3.4-3.6 should be OK on the stock cooler, if you want to go higher, the Xigmatech HDT 1284 and Thermalright Ultra 120 are 2 of the best coolers arround. www.frostytech.com
has comprehensive heatsink reviews.