Hi Keith,
Welcome to the forum :)
To try and answer your questions have a look at the following topics/posts where we've covered some of your queries.
My last post towards the bottom of the page lists what PSU a couple of us use, and some other equipment required.
http://www.dunadd.co.uk/seti/forum/view ... 30&start=0
The U2 and U3 run the same software and are very similar, so you might want to look through the U2 thread:
http://www.dunadd.co.uk/seti/forum/view ... 136&t=2040
1) Is a fan essential
I think a fan is probably a good idea, although the thermal shut off will save any permanent damage its pointless having your machine power on and off all the time.
As per the top link, I'm running an under volted fan which is silent - compared to my pc/graphics card, NAS etc.
2) Do you restrict max CPU%
Nope, mine are set to max speed on demand
3) Does temperature throttling come into play and how do you know.
I hope it will if it needs to but hope to avoid that by cooling.
I've installed CPU Z on my droids where you can look up the CPU load and speed - I have to say whenever I check they are always running at 99/100% load and at max speed although they do drop for a second now and again but this could just be as I'm doing other things or its swapping between units.
My U2 does seem to generate slightly more credit that my U3's so maybe I need to look into their cooling to see if they are throttling?
4) Any good place to source a UK power supply
As per top link.
5) Any other comments
Once you start there seems to be some sort of compulsion to buy more, so be warned!
This thread has our credit comparisons for various projects:
http://www.dunadd.co.uk/seti/forum/view ... 136&t=2078
Einstein is the worst performing one so far, if you're in it for the science then it won't matter. If you're looking to generate credit Enigma is far better.
Any other questions just shout and I'm sure someone will be able to help.
Paul