24GB DDR2 ECC RAM - £16 (I was VERY lucky to get this one) 2x Xeon X5450 - £22 Intel S5000PALR Motherboard - £30
These specs are roughly equivalent to a 2008 mac pro, which costs around £500.
http://www.ebay.de/itm/HP-Z800-Workstation-2-x-Xeon-6-Core-X...
The Lenovo Thinkstation C20 is also a good dual Xeon choice if you can live with a 24GB RAM limit (it's also very compact).
There's also specialized resellers like ITSCO.de and ESM-Computer in Germany (they ship EU-wide).
For ~$1000, a dual X5670 workstation with 12 ECC RAM slots, I think it's an excellent deal.
If you want something newer, the Z820/Z620 are a good choice, they should start rapidly dropping in price soon enough.
These computers are designed extremely well (the 800+ Watt PSUs alone are eons ahead of anything on the consumer market), but even in the rare cases they break the parts are cheap (a replacement system board for ~$100, older but still powerful Xeons also drop in price insanely fast).
I don't know why people buy new midrange PCs for the same price anymore, unless they need the latest processors :-)
The high end X5670 or X5690 should outperform all but the top first gen Xeon E5.
It uses more power at idle, but the upfront cost is almost halved at the moment...
I'd say you should get a unit with the highest end processors you can find (it will end up way cheaper than upgrading them) and preferably with the 1100 Watt PSU, the rest can be upgraded later and you should have plenty of power for anything (dual GTX 9xx for gaming and stuff :-))
Obviously, the upfront cost of the older hardware is still better value when ignoring power consumption, but the v3 will also support much more and faster RAM, PCI-E 3.0, USB 3.0, and other newer technologies which will matter more as time goes on. You'll also have the option of a CPU only upgrade to higher end E5 v3's or even E5 v4's at a later point in time when those CPU models are more affordable.
The X5690 is comparable to the first generation E5-2640 (slightly slower), and the X5670 is comparable to the E5-2620.
Even the first gen E5 has the AVX instruction set, 4 memory channels (vs 3 for the Westmere-EP) and a faster QPI, so it might have better performance in certain applications.
v1 and v2 are socket compatible, but v3 is not socket compatible with the earlier versions, but will likely be compatible with v4 (Intel socket changes usually happen only every other generation). I'd suggest going all new with even a low end E5 2620v3 if you can, which you could for everything other than the GPUs for under 1700 (based on a Supermicro workstation build).