Well I am not 100% the 9600GT will fit because it is a relatively long card. But if you look at it, it is more narrow which makes it a low-profile card.
a 350watt power plant is not very expensive save up a few pounds and get a tougher ps with your video card. I am pretty sure the dell ps can be taken out and swapped witha better one.
I've got the HD3650 in with the normal power supply - it works great.
I was running spore on mostly low settings an now have everything on high settings running perfectly smooth. My vista experience rating went from 4.2 up to 5.2 which seems pretty reasonable