As to the OOF server, its hardware is a workgroup class server. The hardware it runs on - HP Proliant - does far more that run a poxy website 
I never for one minute suggested that OOF was a "poxy" website, and I apologise if this was the impression you got from my previous input.
As it's an HP Proliant, I can tell you it's whacking a fair bit in the power stakes though (even at idle) 
I said it was a poxy website, not you

. And it is really. Nothing particularly demanding about it, except excessive disk i/o
As to the modern Proliant ML (more power efficient than DL) G5 series, they can easily hit 100W. HP's own calculator, that seems to overestimate (not that I would recommend it, as I reckon I caught something nasty from it about 6 months ago

) shows it is possible.
When I first got the server I did measure the power feed to UPS to just over 1/2A, so not far off 130ish W (my AC theory is too rusty!) - thats the server, the routers, APs, network switches, phone system, and the VoIP adapters. That was with the server idle though. If I get time, I may do it again.
Hence why all servers, no matter on the size, power requirements are critical. Hence why workgroup servers with a simple set of 2 or 3 disks can meet the 100W barrier with ease if they are not too busy.
I would argue that it a lot of it depends on the PSU design as well.
Absolutely. All the major server manufacturers are using decent PSUs now, paying particular attention to energy efficiency for the reasons previously stated
How do they reduce power requirements?
At the CPU level, Intel has made big inroads with its new architectures since abandonning it's NetBurst architecture.
Intel have been at the forefront of power saving technology for years, that is no secret.
AMD are following suit, but fell a bit behind when it took its eye off the ball to gobble ATI.
With all due respect, AMD have been chasing Intel for years in both pwer saving and performance......and they have never quite managed to catch Intel.
AMD are not a million miles behind, which is good, as it keeps Intel on their toes. Not sure AMD have the strength to keep up, as finacially they are not great, and since Core2 and gamers changing their alligence to Intel, AMD is losing its core followers.
The other big power hungry device in server class systems is the disk system. Pretty much everyone has gone over to 2.5" disks, initially derived from laptop technology.
Now that's where you have to swallow on your earlier statement IMO......a full on server powering up 4x 3½ disks running at 7200rpm and pulling less than 60W is (while not impressive), something to think about.
Still unimpressed, as my original statement was a small Workgroup server could get close (and also offer the extra flexibility)
I am, obviously, very interested to hear of your concerns about YaBB's security - please PM me if you have any info 
Make no mistake, If I can be ar$ed to look for holes (and I'm sure I'll find them if I look hard enough), then you will be the first to know (via PM of course).
If you really want peace of mind, then scrape another £100 together and migrate to something like vBulletin....I'll quite happily talk you through the migration process so that no info/posts/etc is lost.
If we were to move to other software, vBulletin would be a contender. But as soon as you start paying (even modest amounts), there is an awful lot of choice out there.
Plan currently is to wait and see on how YaBB3 perfoms. Currently, mostly happy with the security and the flexibility of YaBB, just frustrated by its (lack of) scalability.
As to the hosting company giving out a load of FTP passwords, I assume you mean the hosting company for our images server?
Not quite, I'm talking full access to the servers themselves......again, that is not the sort of subject I will divulge too much on in an open discussion.
Reason we continue to use them is because of the low cost hosting (and OOF could not afford to go to dedicated server for the images server), they seem to be one of the best with regards to openness and support. And a million times better than the wasters we used before 
I could probably suggest quite a few others, but again, that's not really something for an open discussion IMO.
I have a reasonable amount of time for this host, as they have been good to both me, and to OOF. Yes, in the past, they have made some pretty significant 'oversights'.
However, only the images are served from there, and if somebody did want to hack the images, thats not really any hardship from security of OOF view.