I used i9 PPT Template System on MBA Business Competition, i9 is a great template system that fit your needs. Beautifully designed, tons of supporting art, and flexible.
You can get the template system from Graphicriver for $15
i9 Template System
Introducing i9. The most comprehensive and functional template system ever created.
Slides. 22 templates.
Dark and light versions of each
Custom gradient bitmaps – JPEGs and Photoshop file so you can make your own