Welcome to Multiboxing, its a lot of fun.

A pally plus 4x shammy team is a good team to start with. The more classes which are the same, the easier it is to set up your macro's and hotkeys. The pally is a capable tank, and probably the easiest to use as a boxer. The shammies have good synergy both with a pally and with each other.

I have no experience with HotKeyNet, but really like Keyclone. From what I've heard, there are a lot of programs for software and hardware boxing which players really like. In the end, it becomes whatever your preference is. The biggest advantage, from an exclusive features point of view, for Keyclone over competitors is Hotstrings for the Leaderless, Focusfree, Targetfree assist system. Keyclone can broadcast mouseclicks, but I've found it easier to just click each window for those few effects which require mouse targeting AoE's.

An addon by Jafula, called Jamba, can be found in the Macro/Addon forum. It is amazing for boxing; I cannot recommend it strongly enough. Once you open the quest window with each toon, the slaves will follow the active toon's selections, which includes going through dialog or accepting quests. If the leader takes an escort quest, the slaves can be configured to auto accept the quest too. You can have the slaves auto select the same loot as the master for quest rewards or to not follow the leader's selection in the case of different classes. It can strobe follow every second, which greatly reduces your chance of losing a toon. If the flightmaster is open on all toons, the slaves will choose the same destination as the leader. A very useful addon with a ton of features. Jafula gives excellent support and incorporates suggestions into future builds. Highly recommended.

I'm running five clients on one screen, but a ton of players on these forums run two, three or more monitors. I'm not really aware of issues for running warcraft on more then one screen at once, but then I haven't personally attempted to set this up either. I know Keyclone has a very nice "Picture in Picture" feature. Basically you will usually have a central window for the active toon, and peripheral windows for the others. Each of the windows can be given a hotkey, which when clicked will swap their window for the large central active window.

8 gigs of ram will definitely help your computer a lot, towards running five clients smoothly. Generally speaking, ram is the biggest bang if you don't have enough. Hard disk access time can really help too; look into Solid State Drives (SSD's) if you have a fair bit of money for upgrades. With or without an SSD, SymLinking your wow folders can greatly improve your performance especially in cities. Essentially a SymLink is a sharing of information between the folders; you have a main folder with everything (generally don't run any clients from this one, and only patch this folder) then a copy per toon which contains everything except Cache, Data and Screenshots. Then symbolically link those folders from the full install to each of the ~10mb wow copies. Warcraft will load the information into memory once, and then the other copies will use it from memory rather then from storage. There is a script for SymLink on these forums, but honestly I found this to be much easier: http://schinagl.priv.at/nt/hardlinks...kshellext.html.

Quite a few people have pointed out, you can often build a second computer capable of running a couple copies of warcraft... for less then it will cost to upgrade your main machine to run five copies smoothly. Depending on what you want, its worth checking out. There are people with a lot of good advice on this site, and the technical forum is a good place for these kind of questions.

I've found for macro's, you can set up a lot of them to be something like this:

#show Lightning Bolt
/cast [help] Lesser Healing Wave; [harm] Lightning Bolt

Your one macro will now perform two functions, on a single click, depending on if your target is a hostile or a friendly. You still have all your modifier keys (Alt, Ctrl, Shift) available and never need to make a conscious choice of which spell to cast for the default click. You can double up a lot of offense abilities with support/healing abilities. This basically doubles your number of macro's.