I'm building myself a WHS system, and I've got an old Prescott 3.2GHz CPU I could use. Now I know that the TDP is high for Prescotts, but as WHS spends most of it's time idling it makes more sense for me to compare the idling power consumption of this part with a modern cpu like a E5200/E6600 or a older 2.8GHz Northwood CPU I've also got in another system. So far I haven't been able to find this information, making it impossible to calculate the running costs of such a system for 24/7 per year.

A E5x00/E7x00/E8x00 on a P35 board or a Phenom 2 will draw a lot less at idle than a P4. You could probably put a E5200 and P35 build on a 120W picopsu, depending on you hard drive requirements. There lots of builds that are idling at 50-70W. If you know what you P4s draw, you can use the $1/watt*year rule of thumb for comparison.

Out of frustation I went and bought a plug-in power/energy monitor to measure my current WHS headless system, consisting of:

P4 3.2GHz prescott
2x512Mb DDR ram
875p/ich5 based board
1 ide hard drive
4 WD 1tb green drives
1 ide optical drive

This system idles at 95W (16W of which are the WD green drives), and , so at 15 pence per kwh, that's a yearly cost of @ £125 (if run 24/7). If I take your comments that a system with modern components would only use 50-70W, the running costs would be @ £65-£92. The cost of upgrading my current system would be @ £160 (e5200 + mobo + RAM), so it would take 3-6 years before the energy savings vs upgrade costs break even.

This wouldn't cost £160
http://www.cclonline.com/product-info.a ... id=mbb0015

Or this lot from Ebuyer, with free delivery:
1 MSI K9N6PGM2-V Socket AM2+ GeForce 6100 onboard VGA 8 channel audio mATX Motherboard 148025 329 in stock £35.80
1 AMD Athlon 64 LE-1640 2.6GHz Energy Efficient Socket AM2 1MB L2 Cache Retail Boxed Processor 160949 236 in stock £29.90
1 OCZ 1GB DDR2 667MHz/PC2-5400 Memory Unbuffered CL5(5-5-5-15) 1.8V LIFETIME WARRANTY 105972 54 in stock £10.00
Cart total inc vat: £75.70

Something like that with a decent PSU would be able idle at less than 40W, but then there are all those extra HDDs...

Pity that mobo only has 2 sata ports. If I paired it right down I could get 6 sata port mobo + CPU + RAM for £90.

ah I never thought of that. Two doesn't seem like very many, even my super-cheapo Biostar one has four. Still, why do you even need that many hard drives?

I have something like 12Tb of media, the vast majority is mpeg2 based (TV recordings, DVD). As it stands I have to cherry pick what I load onto the server's 4tb array.