Sorry, didn't read your post carefully enough. It comes with a PSU you say? Well, just b/c it's higher rated it shouldn't "damage" your computer. I am using a 450 W power supply (generic brand) on one of my old computers (Pentium III). That thing probably draws like only 200 W max :-\..
The thing with generic PSUs (like the one in ur case) is that they are not very good quality. The darn thing could surge and fry your components and such. They don't not very "safe". I'd go with a good brand like Antec, Thermaltake, OCZ, Enermax, etc.
One of my friends fried his whole system recently b/c of a crappy PSU. I suggest that you either get a different case without a PSU and save some money - or get a good nice case with great cooling and NOT save some money
OR just throw the crappy PSU away and get a good brand.