Is this: http://www.pcnextday.co.uk/products/ProductDetail.asp?ProductCode=2518-1536
If so then you'll need to buy a new PSU if you want to power a card like the HD5870. The PSU has most of its power on the +3.3V and +5V rails which isn't much good for todays PC's. Modern PSU's have almost all of there power on the +12V rail(s).
It does have 2 x PCI-E 6 pin connectors so it might power a GTX460, However I wouldn't risk it, Never heard of Zoostorm before.