[Serusers] Ideal server

[Nicolás Lagalaye Falcionelli]

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