AC is very flexible. I have run it on everything from a pentium II at 400 MHz (with Windows 2000, which it is no longer totally compatible, but uusually works) to a quad core AMD phenom with Win7.
My office is populated by win xp pro machines on a peer to peer network. Most of the machines are in the 2.4 to 3.0 MHz range. Most have between 512 and 2K of memory. I actually upgraded last year when I bought refurbished HP computers for about $100 each at a site called pacificgeeks.com (I have no financial interest)
I did add more memory to most and it is appreciated.
I run a gigabit network (most of these had it built in) and it made more of a difference than the memory.
You can spend more and use Win 7 and possible reliability. You can have a real server and network vs. peer to peer, which has a number of advantages. But this works fine for me. Each office has about 6 computers. I have the main computer dedicated to AC, it has 2 G memory and a large hard drive and holds most of the other documents for the practice. It is connected by a KVM switch to another computer near the front desk.
