Pipakin hit most of the good points, so no need repeating them.

Required hardware is not too drastic and other than a good computer and large monitor, nothing else is required. Many people have put together systems for under $1000 that can run 5 accounts. For $1500 I just put together a screamer, yet others can spend thousands just on monitors.

Two parts I can recommend: Q6600 and 8800GTS. Look over the Hardware forum for many many discussions of this exact same nature.