I have that Antec Power Supply. It was enough to power a GF6800U for me, but by no means is it superior to all it's competitors. Lately some people complain that Antecs quality may be slipping, but I can't complain about my 1-2 year old True430w. IMO, Antecs are good, but unless they come in the case, I'd buy other brands like Fortron Source.
Anyway, as Crash said, most powersupplies will definately require you to notch out(cut) the sheetmetal. But I see a plain Fortron 300w that looks like it would fit exactly assuming the screwholes match. Looking at Newegg, it seems Fortron also has a plain <A HREF="http://www.newegg.com/Product/Product.asp?Item=N82E16817104973" target="_new">550W</A> that would fit without notching your sheetmetal for a power switch. 550W and $95 may be overkill for you, but I am not seeing that Fortron has anything inbetween the 300w and the 500w. The quiet Blue Storms are nice (I upgraded from an Antec True 430W to a BlueStorm 500W), but it looks like while close, some notching would be needed.
<A HREF="http://service.futuremark.com/compare?3dm05=658042" target="_new">3DMark05</A> <A HREF="http://service.futuremark.com/compare?2k3=3781954" target="_new">3DMark03</A>
<P ID="edit"><FONT SIZE=-1><EM>Edited by Pauldh on 05/07/05 09:37 AM.</EM></FONT></P>