Should I get a Dual-Rail PSU?

Just found a Quad-Rail for £5 more, so I guess my question now is Single, Dual or Quad Rail?

You should go with a quality PSU, no matter how many rails it has. Some people claim having 1-2 rails with high Amperes is better but I believe if you get a good brand PSu (like Enermax, PCP&C, Thermalright and many more). THere are calculators where you input your build details and give you a watt estimate. Based on that go out and get one and don't be stingy!! The PSU is the most essential unit in a PC!

Don't worry about the rail business.

Just check whether the PSU is of a decent brand and meets your needs.

BTW what are the specs of your rig?

Multi rail psu divide their power per rail. You can't get all the amps your psu offers on the same rail. Instead, it's divided between them.
Theoretically, single rail psu are better for you don't have to wonder if your amps demand is higher than what the rail you are using has to offer. However, if you are no planning on lots of heavy parts (2x5970, plus 980OC), it won't make a difference.

Currently I'm looking at:
AMD Phenom II X4 965 BE
XFX ATI Radeon HD 5770 XXX Edition
At least 4GB of 1600 DDR3 RAM
Then just the usual Optical, HD etc.

Here is a link to an excellent article that explains mutiple +12 volt rails:

http://www.jonnyguru.com/forums/showthread.php?t=3990

That is EXACTLY the kind of thing I needed. Thanks.