Duff DIMM was the problem. The PC is working fine with just one 4GB DDR2 DIMM right now. Problem is DDR2 RAM is now so pricey that it's cheaper to upgrade the MB and replace all 8GB (the working 4 and the duff 4) of DDR2 with DDR3 than it is to buy a single 4GB DDR2 800 DIMM. Grrr...
I forgot to add that I bought the Intel Sandybridge i5-2500K Unlocked Core i5 Quad-Core Processor CPU (3.30GHz, 6MB Cache, Socket 1155)
There is also the cheaper i3 2120 Sandybridge Dual Core 3.30GHz Processor CPU around £90.00 or so and cheaper still the Intel Pentium G860 3.0GHz Dual Core CPU which can be bought for around the £50.00 mark.
Yeah, but this was the left-overs from the left-overs PC. Not going to spend any more moolah on it unless someone else (i.e. 'er indoors whose office it is intended for) coughs up.
In April I upgraded my primary PC to an i7-3930K 3.2 GHz hex-core with 32 gigs of RAM and a shedload of RAID'd SATA-3 storage. The leftovers from the upgrade were added to a cheap new motherboard to build an Q9650 3 GHz quad-core media PC with 2 terabytes of RAID'd SATA-1 hard disks and 8 gigs of RAM, and the left-overs from that got used to build the PC in question.