Memory, memory, memory. Doesn't matter how fast your processor is, if you have less that 1G memory on XP or 2G on Vista forget it. We are running 3 Laptops at a time over a wireless g/broadband connection. The 2 with XP and 1.5 G memory work great, the one with Vista and 1 G was super laggy until we added another 1G stick to it. None of them have a processor faster than 1.7ghz and all have integrated graphics chips. We do try to play on quiet districts though and can immediately tell if a system scan or windows update kicks in while we are playing.
It is so easy to add memory and if you order it straight from Edge or one of the online stores it's not very expensive. Not just POTCO but everything runs smoother.
