Quote - It absolutely has to be 300dpi. 72dpi works only for screen renders or low-quality small images. It won't work for professional print jobs.

 
Unfortunatly i´ll need to disagree on this. If the client demands 300 dpi, there´s no question. 
But i actually work in a printshop, printing out high quality art posters among other things, and 72 dpi works perfectly fine in most cases IF you render out large. 
You simply wont see the difference in the end result.

Unfortunatly 300dpi have become something of a mantra in the business and everyone wants it, but there are actually very few cases you really need it for printing purposes.

Nuff about that...

Are you using a lot of highres textures? If you do, try changing some of them and see how much difference these make. Your windows shouldn´t make that much difference unless they have a very high reflection value.

Rudolf Herczog
Digital Artist
www.rochr.com