You can actually save money buying some of the parts second hand.
Personally I think it is OK to buy Case, RAM, DVD/RW & Motherboard second hand. In fact I would even consider second hand CPU and GPU but I know lots of people would disagree. Normally would buy PSU and HDD new.
By doing second hand you can build a much better PC but there is more risk.Would you consider second hand or you only want new?
If only new what do you want PC for and what parts do you need?