I just bought a new AMD sempron cpu and it came with a heatsink that had preapplied thermal paste. I installed both in the motherboard that I also bought though I haven't run it yet cuz I'm still waiting for the ram. My concern is I removed the heatsink to check on the cpu and placed it back right away. I read that this unevens the paste and will cause overheating problems. SOo will I have to apply new thermal paste?=/
I plan to unlocks its other core but I don't think I'll be overclocking it.
You might have to put on new thermal grease, but you might not. The stuff the OEMs put on the bottom of their heatsinks is a thin pad-like material that isn't sticky or gooey like consumer thermal pastes. If you powered on the CPU and the paste got warm, you would absolutely need to replace it. But, if the paste looks more or less intact and there aren't holes or dimples in the paste from your CPU squishing into it, you might be okay. It really all depends on how much force the heatsink was clamped on with. My guess is if you fully clamped down the heatsink and latched it, you probably will need to replace the thermal grease.