Should be no problem
I have a 1gb Sapphire 4650 fan cooled low profile in a Dell Optiplex with
a Dell 280 watt psu.
I have even mildly overclocked it and ran PCMark and 3dMark03 with no
The HD 4000 series is famous for low power usage.
I have a Pentium D 3.4ghz and I run BioShock 2 at highest settings at 1920x1080 res with no lag.
As long as your psu is a reasonable name brand you should be fine.
Also as long as your not running 2 hds,2 optical drives and multiple slot cards you should be fine also.