A new big Server or adding a little one beside the old?

Considering to X+Y=3DZ equation, there is 2 choices facing us.In = equation you can think Z as the sum of X and Y little servers which make = Z (big server). According to this situation i want get your advices for = custumization of our system. 1.) We want to buy a new little server(Y) (HP-UX 4 CPU,5 GB RAM) for = assisting our old server (X). In this option we think to separate this = two servers as one will work as application server and one will work as = DB server, in clustered mode. 2.) For second option, instead of adding a new little server to our old = server, we think to buy a new big Unix server,possibly with 8 CPU.And = both Application and DB will run on it. Our current configiration is HP-UNIX with 4 CPU,5GB RAM which runs = Informix IDS 7.35 under Baan IV.We have 750 concurrent users accesing to = server,and expect to have new 50 users in 1 month.Currently we are = having storage and CPU bottlenecks . Considering to user and data growing which option should we prefer to = choice? Thanks in advance