i also have a hp slimline....
the power supply is only 160w
the galaxy geforce 9600gt requires 400w
did u upgrade ur psu??
if u did can u tell me where to get one?
the galaxy geforce 9600gt states it's low power @ 300w...which would still require a PSU upgrade. i'm only using it for a PC media center, and read the stock PSU would work ok for this application. so far so good.
as for threads on PSU upgrade, here's a pretty good video http://www.youtube.com/watch?v=D8Y2PPMiiB0