[Serusers] Ideal server

I have to buy a server to host SER, serweb, sems... It will run the router, the PSTN gateway (using E1 cards), voicemail, web inteface, and billing services. Though number of clients will not be very big I want it to be strong and stable. I must decide the hardware characteristics: I actually do prefer AMD processors rather than Intel. But, what king of processor I should use? How many RAM? About Hard Discs... Will a couple of Serial ATA 120GB discs do? Finally, what Linux dist should I use? Red Hat is strong and enough prepared so as setting up ser, sems, serweb and modules not turn into a headache? Many thanks, Nicolás Lagalaye Falcionelli