yes, pick up an extra fan or two---- Minimum IMO, 1 fan in the front of the case, the CPU has its fan, one 1 fan at the back of the case. Obviously, the PS has its fan. Do you need more fans?
Can it hurt? --- it can make the PC run louder
The raid questions depends on what you want to do with the PC.
Do you "NEED" raid? There are different kinds of raid, which all depends on what you want to do
I have Raid 0, which is 2 hard drives acting as one drive --- very fast, exspecially since I have two 10,000 RPM drives
IMO, buy the best memory you can afford.
I found good prices at