Do not upgrade to a 8600GT if your paying for it. Refuse to buy it, whatever.
I had that card around 3/4 years ago. It was a SEVERE let down, barely an upgrade from my 6800GT.
It was only around £40 more for the 8800GT at the time (AMAZING card for its cost, still not redundant yet. Only recently upgraded from the 8800GT to my GTX260)
If you want a cheap/powerful card. Then look at the ATI 7750. http://www.ebuyer.com/344116-sapphire-hd-7750-1gb-gddr5-dvi-hdmi-displayport-pci-e-graphics-card-11202-00-20g
£84 here (Not including delivery). It also requires only a single power connector, so its unlikely to need anything below an average PSU.
By the way, that PSU of yours does look pretty tame.
Personally thinking of upgrading to the GTX6xx from Nvidia (looks so goddamn powerful compared to ATI), which is the main reason I wouldn't think of upgrading to this ATI card.
What are you upgrading? Is it the intregated graphics in your tower pc?