The 6800GS is by far the best budget card out of there, certainly out of the ones you listed.
The Hiper's are nice, but the 580W will set you back more than the Tagan.
As for Overclocking, it's simple enough really, and the chip you are getting (presuming the 3700) is a good overclocker, so yeah, keep it in mind.
www.scan.co.uk is also a very good UK based retailer, and if you do come over here for Uni, you should give them a look, I've bought thousands of pounds worth of stuff from them and never had troubles, as have thousands of other people.
((Zink, it's strange, opinions on Overclockers UK are so mixed - I mean I've dealt with them before and I have to say I was very impressed - I also love their site design, however a point to note is that I've never had to RMA / use their tech support.
))