inside the fan and on the board, old inefficient thermal paste can cause overheating. Insufficient RAM may also cause overheating.
Open the laptop and use some "canned air" to blow out the dust from inside the fans, the cooling slot and off the logic board. Do not touch the heat sink at this time, as it's a rather touchy DIY job. Reassemble the laptop but do not screw down the top case just yet. Perform tasks that used to overheat the box. If overheating no longer occurs you're good to go.
You should max out the RAM for your machine.The MacBook Core 2 Duo shipped with 512 MB or 1 GB of RAM. If your MacBook is still running with only the stock RAM, upgrading will provide a dramatic performance boost at minimal cost. Upgrading to the maximum (3 GB for Core 2 Duo, 4 GB for Santa Rosa or Penryn) can be done very easily.
If no significant change occurs in the overheating you could go the next step and replace the thermal paste. Take care with the disassembly/reassembly of the the small connectors, they are easy to damage.
As far as being obsolete goes. IMnsHO if the computer still performs the functions for which you purchased it, there's no need to change up to a newer box. If/when you purchase a piece of software or hardware that requires more power than you currently own, that would be the appropriate time to upgrade.
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