Dell PSUs are way underated.
I use and fix Dells extensively
Currently I have a Sapphire HD 4650 in a Dell Optilplex 745.
The PSU is 280w.
I pin modded and OCd my CPU from 1.8 to 2.4 ghz
and also OC my GPU.
Everything runs smooth.
In my old Dell Precision 450 I had multiple optical and HDs plus GPU and Soundcard
and according to PSU calculatorhttp://extreme.outervision.com/index.jsp
is said I needed minimal 475w.
I ran that system for years with original PSU and it is still working!
Dell PSUs if I got it right are Fortrons which is a good brand name.
I would stick with cards that dont need external power if you do not have the plug on the PSU.
You should be fine.