I use 2 computers, one runs 4 the other my main. I didn't spend all that much on my slave box, it's a E8400 with 4G RAM, 32bit Vista and a 9800GT. I have zero issues with it so far (of course, I am still <70 and Outlands is a ghosttown).

Now, I am also focused on PVE. I think if you were to PVP, you would like to have the fastest response possible. That would be either 5 computers or 1 really good one. I'd rather use 1 and not deal with the power consumption and heat and extra hardware. In addition, with 1 box you have to use software KVM. With 5 boxes you can go the hardware route, which is supposedly a faster response. Again, faster rsponse is probably better for PVP.

I recently just built a new box for my slaves, only due to my main computer (which is over 3 years old) having a problem - which I later fixed, go figure. The new box is a Q6600 with 8G of RAM running Vista64. I will keep my 9800GT for now, I doubt I will need to upgrade it. The new box cost me $700 CDN (probably $600 USD) [also I have $100 in rebates that I will get someday] (CoolerMaster Case, a Gigabyte P45 MoBo, OCZ DDR2 800 RAM, Corsair 650W PS and Intel CPU). I have not overclocked it so am using the stock CPU cooling fan (for now).

I don't think you need to spend even close to $5k+ for five separate boxes. That is way overkill for WoW...it's not that resource intensive at all. I'd guess more like $300USD or less plus monitors for each box.

[ As a side note, something I learned recently is that the max RAM speed you need is the FSB speed /4 X 2 (for DDR2). So in my case, the Q6600 is 1066MHZ, which is a 266MHZ FSB, so to match the RAM speed 1:1 I only needed 533Mhz DDR2 (if you overclock, it is going to change this). I have DDR2 800 since it's cheap and readily available but won't use it to it's full potential unless I overclock my CPU (my MoBo can go faster than 1066Mhz). My point is, don't get faster RAM that is more expensive since you normally don't need it (like I almost did - it was on sale, but still slightly more $).]

So, you need to decide:

1) PVP or PVE - what is your emphasis?
2) Software or Hardware KVM