Overheating issue in left top corner

I am not closely familiar with your model Mac but on my notebook (late 2008 MacBook) that's where the CPU is located and that is what generates heat, particularly if you are doing CPU intensive activities such as gaming or processing video. With warmer summer temperatures it could be running hotter. There are various utilities which let you monitor temperature sensors in your computer. Hardware Monitor does them all but is a commercial utility ( $ ). I constantly run Macs Fan Control which displays temperature for some of my sensors. It does not do CPU temperature but I did a comparison of the diode temperature which it does display and it was was showing the average of the two CPU temperatures displayed in Hardware Monitor. First I made sure my fan and cooling vents were clean and I opened the computer and used canned air to blow out the dust. I felt with video processing it was still running hotter than I liked so I used a utility called CPUthrottle to cut back on CPU usage by my encoding application. Of course encoding takes longer but I am happier with a cooler machine.

I don't know about the RAM so much. I would make sure you stick with a reliable brand that is known to work with Macs such as Crucial or OWC (Macsales.com). Macs can be really sensitive to anything but top quality RAM.

You can check process in the Activity Monitor utility. Note that % may be over 100% because that is percentage per core. For example when I have video encoding I have it set to 100% but since I have a Core 2 duo that is only half of the 200% total I have available. Normally my encoder would gobble up anything I am not using for something else meaning my CPUs were working full force all the time and getting hot. Now I have the encoder throttled to 100% it leaves the other half free for other things. Now my computer runs at 150% under normal use and my diode temperature instead of being about 90°C is usually around 75°C (105°C is automatic shutdown for this model).

For more convenient and ongoing monitoring you may wish to use a utility which displays CPU usage in the menu bar. iStat Menu is a good utility that is now payware. MenuMeters is a free alternative: http://www.ragingmenace.com/software/menumeters/index.html

Check your processes. On occasion one may have a runaway that has crashed but is still running. Restarting the computer helps.