Dell Inspiron 620 - Power Supply Upgrade issue

Anyone can offer some advice please.

Trying to upgrade my son's Inspiron power supply so we can fit Radeon 6670 graphics card which needs additional power through a 6 pin PCI express socket which the standard Dell PSU cannot provide

About 6 months ago we tried to upgrade the power supply with a standard higher powered ATX psu from PC World and despite having all the correct power connectors the machine refused to boot. Front panel switch did nothing to make it burst into life. I ended up using the PSU in another Dell PC and we gave up.

Well we've decided to revisit and I bought the following PSU from ebuyer.

http://www.ebuyer.com/390639-alpine-700w-blue-psu-12cm-fan-4x-sata-alpine-blue-700w

Again exactly the same symptons as before. All installed - all cabled up but the damned PC will not start. I jumpered the ATX power supply to force it to power up - the hard disk starts to spin - but no boot - no screen display regardless of intalling the new graphics card or using the standard on board graphics card.

PC is out of warranty. Is there an issue with the Dell PSU being proprietary and therefore why an off the shelf ATX power supply won't do the job.

Getting to the point of throwing the machine out and just reusing the memory / hard disk / DVD and processor on a new PC ( but it won't be a Dell again ! )