You could hook up a 1000W PSU, but if the system only requires 300W to run, it will only use 300W.
Use this to get a baseline idea of how big a PSU your system requires:http://www.thermaltake.outervision.com/
I like to have ~100W more than I need, to account for any future additions.
I also only use Antec PSUs. I have never had one fail me even in the slightest way.