Right. I've just upgraded my system (~£800). Here are my amateur learnings

Old
Core2Duo
8meg ram
8500 Nvidia 250meg

New
I7 2600k
Z68 motherboard
16gig RAM
880 PSU
560TI overclocked EVGA

First, make sure your DVD drive is compatible with the new motherboard, otherwise you'll suddenly realise after 2 hours of installation that your lovely drivers on the lovely boxed DVD's are completely useless. Not only that, but Windows won't recognise your ethernet controller. This is where work laptops + usb stick + internet are handy.

Second, if you bought a fancy CPU cooler, don't install motherboard/graphics card/everything else, and then realise you need to take it all apart because you need to screw something on the back of the motherboard.

If there is a large thud as you install the stock CPU cooler, sweat and drink a shot of vodka.
If you get a headache, lie down for 30 minutes (it helped me)
If the children start screaming, close all doors
If the wife starts screaming, send her to bed
Don't sneeze over the brand new motherboard
Be organised, so you know which wire goes with which component. If said component doesn't fit, don't force it in and panic.
Be really glad that the new graphics card comes with an adapter for VGA/HDMI.

And finally, when you receive all these lovely components and freed up an evening of getting it ready, be glad you ordered a PSU with them and you didn't have to find a shop near work the next day to buy it overpriced.

By the way, I can't believe I managed to 5 box on my old system. Clean 60FPS makes WoW look absolutely great (bear in mind what i'm used to)

And thanks to everyone who helped and contributed to this site!