My current setup is:

i7 4770K
24GB 1600MHz RAM
Dual R9 280X 3GB Graphics and I can comfortable run 10 clients. My main bottleneck is the horrible internet connection I get to home.

I was previously running 9 clients on an:

i7 2600K
16GB 1600 MHz RAM
Radeon HD 6970 2GB Graphics

With no issues other than me cooking my GPU because of the horrible cooling I had in the case.

I find that with 10 clients i use about 11GB of RAM so I defiantly say 16GB as a minimum for that many accounts.