each: Bought smart in the US: $200 for cpu, $160.00 for a decent board, under $100 for memory and then add in the rest
Good heatsink, GOOD quality PSU,case,etc.
Then factor in cost vs output and don't forget to add in the electrical usage planning on at least 2 years for the machine before upgrading.. That electrical figure is important in the overall cost.

Think 4GHz on the i7-920 not 3GHz, get a good cooler like the TRUE( ThermalRight Ultra Extreme 120) and it's doable..
I do understand your dilemma though.
Nothing sexier than a multi socket system but numbers are numbers.
The other side is it's your money and do what makes you happy.
That is most important.

I'm confused as to what you mean by "bi-processor"?
Dual socket? or dual core?
If you mean dual socket then what I have here is the most powerfull thing out there( sounds like bragging but I'm dead serious)
Look at the list I made in a previous post here.
The Supermicro X8DA3 board can't be beat for stability and dependability for a workstation board and I beleive there is a hack to run two nVidia cards in SLI on it.
http://www.supermicro.com/products/motherboard/QPI/5500/X8DA3.cfm

The only thing I see out there with more potential is the Beckton based machines ( dual and quad socket native 8 core cpu's on socket 1567) that IBM (nos_support) team has running but those are not available till 2012 and right now only running at 2270mhz I beleive.
At their current speeds of 2270mhz and me at 3458mhz I think I can beat them( dual socket versions) in terms of output per day but that may be just wishfull thinking on my part!
It's very hard to compete with a company of that size when you only have the resources of a blue collar individual but I'm trying!

For a single socket machine and forgetting cost I'd go with a top X58 chipset board and one of these upcoming 6 core 980's that will be available in March..
One of those at 4200 on air should do close to 50,000PPD in WCG.
Maybe a bit more..

Movieman,

Great info in your posts! Thanks a lot.

Have you a power consumption figure for that rig?

Thanks

Thank's for your reply Movieman. I have really to think about this dual socket issue.
Now if I add the cost factor I am not sure but it will be similar between the two solutions or even less expensive with two desktops single socket than one (bigger one) with dual socket. The two top hexacore Xeons will be more expensive then the two top i7 hexacores. But yes the dual socket solution has an advantage and that is space, volume. It will consume less space, and this is an important issue.
All this makes me think again about what should be my RA (the sun) device. Instead of having it dual socket maybe I will have it as the only device with extreme overclocking, watercooling etc. After all I never went into that field and it could be fun. Everybody knows that the Sun is very hot. Core temp is in the millions of degrees.