RAM switch is trivial, hardest part for me is undoing the screws on the back. Crucial / Kingston are good, I have used another supplier (forgot name 😳 ) and they where fine too, my point is many decent options available. As on other threads I have done 4+4 and one single 8 chip in another machine to give 2+8=10. Going from 4 to 8 is worthwhile, above that unkess younare very graphics//processor intensive then an ssd is a better bet
EDIT: worth checking max ram as Apple site sometimes understates what you can put it. Is 1066Mhz is expensive you can run 1333 (I think worth checking). If machine is “slow” worth checking ram pressure in Activity Monitor to see if its ram. Worth downloading BlackMagicSpeedTest to cehck hdd speed (35-40 ?, my 7200rpm replacement hdd runs at 100 and an SSD will do 250)