The 12v rail is how many amperes (amps) of 12 volt electricity the power supply can put out.

AW 3_3 Is my build ok :O for a £300 spare machine?

looks pretty good. Go for it

I found what the model for the PSU and it's price:
Dell : DELL 250 W Redundant Power Supply for Dell PowerVault TL2000/ TL4000 Systems : Computer Components : Home & Home Office

That price is for a brand new made by Dell PSU and it's so expensive because it's not something they make regularly, the people on here know what they're talking about. I bought a brand new Hercules 500watt PSU for my computer off ebay for less than $50 (I can't remember how much) and a Hitachi 82gb Harddrive for less than $30. I actually blew two PSU's (both 200w) on my old computer by adding a 2nd harddrive, pcmcia card reader, USB hub, etc (I blew the original and it's replacement)...250 watts isn't a lot, especially with today's pci-e gfx cards (which I noticed your potential board has)

But they seriously would not charge that much for one.