Well, it's Cooler Master, so you can trust it to put out a few more watts than it's rated. I ran a similar rig on a 570W 'Trust' power unit for some time, which obviously was a POS and probably didn't even manage 570W. If you're unsure, you could try this'n
Newegg.com - COOLER MASTER eXtreme Power Plus RS-500-PCAR-A3-US 500W ATX12V v2.3 Power Supply
few more watts under the hood, not too much more either.
EDIT to answer the rest of your question ^_^:
Micro-ATX normally fits in ATX cases (it says on it that it's got MATX compatibility) but ATX doesnt usually fit in MATX cases. The Three-Hundred is one of the best cheaper cases around IMHO - didn't want to recommend a more expensive one since your budget was tight with the components i was lining up for you ^_^
So yeah, if it doesn't break the bank that case will probably be a better cooling solution due to it's exposed grills (better flow).