I intend to build a PC for folding and other distributed computing projects and need advice. I know a quad core CPU will greatly speed up folding, so I'll be getting one (likely an Intel one). My new PC will be left on 24/7 with the monitor turned off. It won't be used for gaming, but will be used for the occasional access to the Internet.

But what about the other PC components like hard disk, motherboard, RAM, PSU etc.? How can I minimise electricity consumption by my PC and still fold as fast as possible? Which should I buy for my future folding PC? I'm really confused here, so advice is appreciated!

Since you didn't state a budget and you said no gaiming get :

any G33 motherboard (GA-G33M-S2H has a DVI connector)

4GB 800MHz DDR2 (2 value kits, 5-5-5-15 timings are enough)

SATA hdd (160-250GB WD/Seagate)
A GreenPower WD would reduce energy consumption
but it's expensive and you don't need 750GB.

You should buy a good 80plus certified 400W~500W PSU.
The Corsair 450W VX PSU is a good choice but quite expensive.

Just to give you an idea what various configurations can do:

RAC
1,030.89 GenuineIntel Intel(R) Core(TM)2 CPU T7200
297.58 AuthenticAMD AMD Athlon(tm) XP 2800+ Linux
2,463.80 GenuineIntel Intel(R) Core(TM)2 Quad CPU Q6600
964.27 GenuineIntel Intel(R) Core(TM)2 CPU 6400
779.53 AuthenticAMD AMD Athlon(tm) 64 X2 Dual Core Processor
149.65 AuthenticAMD AMD Athlon(tm) 64 Processor 3400+ Linux

All are currently stock speeds, the quad is Vista, the others are XP pro.
One Linux is Fedora 5 IIRC the other is an Ubuntu 6.06 server.

I only run SETI/Boinc.
The Q6600 is used for gaming but runs SETI 24/7
The T7200 usage varies but when not used for something else it runs seti 24/7
The 6400 is Seti 24/7. I also use it for financial, office and other software.
The 3400+ is a fileserver and Seti 24/7
The 2800+ is Seti 24/7

The RAC shown is with all of them running 24/7 for over a week.