If you haven't: Get a better HS/F Thermalright IFX-14 or Ultra 120. Get more/better ram, if possible 2x2Gb or 4x1gb. Overclock the ---- outta that thing 8*400 should be reachable and put ram on a 2:3 divider so you'll have ddr2 1200. You might just want to wait for the new ddr3 rams..
You're memory is fine. Raise your divider as Dev stated, and that will raise your frequency. If you have problems POSTing on CL4, then loosen your timings. You will notice almost no real-world difference between CL4 and CL5 (if you really think about it.) What is the big crave for 4GB of RAM? Is it really needed? Granted RAM is at like an all time low, but you really don't need 4GB...