They're all subjective.
1 - most folks are fine with 4GB, however pricing puts 8GB at very attractive levels
2 - You have to go with what you want and are comfortable with. 22" is actually the minimum I recommend to people buying new (widescreen)
3 - You want your PC to remain cool under pressure - ideally no more than 50C on the motherboard, CPU range from 30-50C, and video cards are fine in the 70-90C range.
As Iantdemo said, adequate case cooling makes all the difference.