Jamba is amazing, download it, use it.

If you have 7200 RPM drives or faster, you should be fine. But if you're hell bent on spending money, I'd go with the Intel 80GB SSD drives. Also, if you're running all 5 clients on one box (this is my setup), you only need one video card. Just make sure it's VERY high end. I use Radeon but Nvidia is just as good. I personally feel the most optimal setup is having two monitors. One for your main and another for the 4 slaves. It works great.

As far as Keyclone goes, I can atest to the ease of use and reliability of it. You don't have to create scripts or anything. However, if you go with the focusless/leaderless setup, you're gonna have to create keymap files that can be a headache if you're not used to it. Mouse clicks can be passed to your wow windows using Keyclone and it works reasonably well. No matter what tool you use, passing mouse clicks will always be tricky when you have different size windows for your clients. A WiFi mouse works okay if hardware boxing. Either way, mouse click passing is usually reserved for using some AoE ability, not interacting with the environment on all toons.