I have two decent machines - including one pretty new machine that I put together last fall. It's a quad-core Intel (6600? - can't recall for sure), 8GB RAM, Vista 64, 10,000 RPM drive, Radeon 4850 video card, etc. From others' posts here, I'm assuming that main machine is capable of running at least 4 characters. My other machine is a dual-core Intel CPU, 4 GB, running 32-bit XP.
The 8gb one will run 5 with the settings maxed out. The 4gb will struggle to run 3, it will page ALOT in cities.