Saw in a shop an open air type PC in a shape that made me think of the LAX airport or similar made by Antec. This Skeleton has a huge fan hovering over the top, quite silent.

Ah yes, here's an image:

I do not have experience with servers but I researched the topic and it all depends of the context. I suppose we talk here of machines designed exclusively for crunching 24/7 no other work.
1) Where and for what purpose. I will consider the case where you are at your home and you do this for a personal free contribution and not in conjunction with work related things or business use.
2) What is your knowledge, and personal competence with servers and their associated software. Have you experience with datacenters, server farms etc...
3) What is your space available, power supply, ventilation, cooling, and noise (do not forget you are in your family home).
4) How much free time available to do this if you are a working professional, and say not a retired (you lucky guys) with much free time available.
5) And last but not least how many CPU's you plan to have: 5, 10, 25, 50, 100 or more.
6) Is getting maximum crunching performance important or not.
8) What access facility do you have to buy new or used equipment etc. etc.
7) Last but most important: how much are you ready to dish out.
Point 7 will also be the guiding element to set up a compromise on all the other points.

After having looked at all that I found that the compromise that suited me best was not to take the server route but to multiply identical standalone desktop rigs. I am here in the range of about 10-15 CPU's or 80-120 threads.
I choose only standard components that you can find everywhere on the general public market, which means easy and quick availability at the shop nearby.
I insist on selecting the best possible best possible quality and here I have experienced over the years, and I have set up my personal standards. The two key elements are a top of the line mainboard that has overclocking and upgrading potential to last for sometime, that is robust and reliable at a reasonable cost. This means for me single socket mainboards. Again the best CPU's also on these same terms and for performance.
I have no experience with servers nor the time to learn about them. The advantage of the desktop solution is that if in the future I want to or have to do something else and stop crunching then each desktop is immediately usable as a general purpose PC for work, home, game etc..

I will do an exception for a double socket desktop I plan for this summer, but it will be again as a standalone two socket machine.

If you look at production vs cost and I mean not just the cost of parts but also add in electrical usage over a 2-3 year period of time then you'll find out that the best bang for the buck is a single socket good quality motherboard.
At this moment in time I'd go for a i7-920 on one of the better boards, 3x1 gig of good memory, good heatsink so that it will handle the heat dump when you over clock it.
Even with a mild OC to 3200( and 4000 is very doable with the right parts and some knowledge) that machine should do over 30,000WCG PPD or if you wait till the new 6 core Intels come out get yourself a cheap L5640 and that will amaze you with what it will do..
OOPS, did I say that?

Oh, this is coming to you from someone with 4 dual socket machines on the project.
They do have their advantages but don't compete in the bang for the buck dept.

Thank you all for your replies much to be considered,
I'll start with Hypernova's (love some of your planetary system analogies to WCG crunching by the way)
I'm going to quote him and answer by old fashioned cut and paste way...

1) Where and for what purpose. I will consider the case where you are at your home and you do this for a personal free contribution and not in conjunction with work related things or business use.

Yes I am at home and I have a furnace that is so loud in the basement that a few computers wont matter (I use headphones for my games helps drown out the furnace and washer and dryer)

2) What is your knowledge, and personal competence with servers and their associated software. Have you experience with datacenters, server farms etc...

Servers not at all, I've put together and upgraded desktops before still seems like I could fuddle around and do that...

3) What is your space available, power supply, ventilation, cooling, and noise (do not forget you are in your family home).

See number one

4) How much free time available to do this if you are a working professional, and say not a retired (you lucky guys) with much free time available.

I'm retired military and my wife pays the bills so retirement money is fun money (I work occasionally and throw that money into the bill/debt pot) and less I sound totally irresponsible I often write over my retirement amount for purchases out of the norm (new furnace etc)

5) And last but not least how many CPU's you plan to have: 5, 10, 25, 50, 100 or more.

Was thinking 10 4/6 cores roughly in a server rack.

6) Is getting maximum crunching performance important or not.

Uhm if I can get 30% less processing for less than half the price then I would like to shoot for the 'sweet spot' I7 920 at 200 bucks is a good example of what I would call cheap processor that does decent amount (my alienware gaming rig has one and I can't complain)

8) What access facility do you have to buy new or used equipment etc. etc.

Uh, i'm in Canada so over the internet even with taxes added is cheaper than buying in store.

7) Last but most important: how much are you ready to dish out.

That's the thing isn't it... If I can get a big server case and just add one slice (blade?) every couple months or so, maybe 2 or 3 thousand to start... no idea if that's best way to go

One other Idea, power is 7cents a KW here so desktops might be the way to go.

uh aren't all stoves 220 in the US?