5850 itself is an awesome overclocker. MSI's one isnt. Refference card( i.e. ATI card like Asus f.ex.) overclocks fine due to "unlocked" voltage tables, and those tables have many more voltage steps. MSI 5850 on the other hand, has a physically "reduced" voltage controller, and it has only 4 settings: "0.9500", "1.0380", "1.000" and "1.0880" which is the highest voltage possible on a MSI HD5850 Twin Frzr. As you can see here:
http://img267.imageshack.us/img267/1...kedvoltage.jpg
Edit: Dont think that the moment you install MSI Afterburner, you can overlock the core to 1Ghz right off the bat. You cant. I am using Afterburner myself (newest build) and the moment i set it to 1.1000 or 1.1500, OVP kicks inn and i get black screen and fan on full and have to hardreset the computer.
Anyway, this is not a ATI vs nVidia thread, im just saying, stay away from MSI (stock or TwinFrozr) if ure planning on overclocking. MSI Afterburner does not always work.
Edit: attached screenshot example (removed IMG tags cos the image is big)
http://img39.imageshack.us/img39/2124/msilolvolt.jpg