Mobile i3 is plenty fast already. I would look into sacrificing hard disk space (150-250 gb should be plenty), RAM (3-4gb is enough), processing power (i3 is good) to get a dedicated GFX (just a cheap one) and get a nice screen and speakers. A nice screen + speakers will have a larger impact on usage than raw processing power. On the other hand, having a dedicated GFX does make a difference for high resolution videos like youtube at 1080p while maximized.
I'm not sure what which companies currently sell the laptops with the best displays so you will have to go to a store to check it out. Try to get a glossy type LCD. Higher resolution is better (up to about 1680x1050).
Does your fiance like 15" monitors? My sisters tend to prefer the smaller 13" laptops.
Personally I would go with the $599.99 laptop I linked. It would be more than enough processor wise to run Microsoft Office, iTunes, and Photoshop easily. It also includes a 500GB hard drive which should be more than enough but if it isn't there is always the external option.