But we dont know what case this guy has in his current system.

Looks and quality of a case mean nothing to a degree. If you are going to be overclocking and running serious Hot Temps as a result then yes, airflow may come into it.
I beleive as this guy is asking what components he needs, he wont know how to overclock and the best PC for him is one that works fast out of the box and therefore wont need a £80 case using up his small budget.

That power supply I misquoted on, its actually 550W output and not 500w and using a psu calculator program and all his system components working at 90% load it comes out at 522W. I included an extra HD in that as well as upping the settings a little. It also has the required 24 pin adaptor for the motherboard, and a single 12V rail which many believe is better. He could buy a 600w for another £5.

Interestingly that Dino shop PC only comes with a 400w PSU even when you add that 8800gts to it. I do not think this PC would power on unless the PSU was changed. That is why it seems so cheap.

If the lad had been into overclocking then I would have said go for a bigger PSU to facilitate this.