This question on its own will get you many different answers from different people. The current World Correspondence Chess Champion won his title for engine analysis help with i5-4670k o/c 4.4 GHz, with 16 GB of RAM (2 GB for hash) if that is any help. Of course there are others who have the top of the range hardware using many cores etc. etc. ( and it could be said that hardware will make a lot of difference to how well the software/engine will play) but really it all comes down to two things, how much money (but more importantly) how much time you are willing to spend.