I would have thought you would need a pretty big server but it is hard to say without knowing your application. To be on the safe side I would spec a box which has plenty of potential for expansion i.e. Sun 12K or up. Buy it with a low number of CPUs and then increase the number of CPUs if you have to. If you have nice reseller they may let you have access to kit to test your application and then you can decide which box you need. You may also want to consider a RAC type setup if you need high availability. As for storage go for RAID 0+1.
Your reseller should be better able to advise you on all of this though if they are half decent.