All parts are important. Id say the least important part is the optical drive. Many people mess up their setup with a cheap psu and case. I know many friends who went with a cheap psu or case, since they said 'a case or psu wont give me extra fps', and now they suck in the summer with the temps due to now enough airflow, or simply their psu give up.
So I'd say, if you make a new PC, don't save on any main parts, or otherwise you will have problems with it. If not right away, in the really near future.
As for a personal example, I bought a 460W Enermay PSU around 6 years ago, and I'm still using it in my new system. Spent my pants on it, but It was worth it.