Worth getting thermal paste? Ran 20 youtube videos and got high 70 (degree C) CPU temperature and 80% RAM Usage

Getting Thermal Paste will surely not solve your problem. If you upgrade your cooling, i.e., get liquid cooling or add additional fans, only then the heat will dissipate properly.

Your motherboard probably has 2 RAM slots (1X2GB & 1X4GB) and so when you buy RAM, you'll have to replace the 2GB one. Also make sure that the RAM is dual-chaeled (i.e., having two RAM boards of the same type) so that your computer can read and write to the memory at the same time.