Like others said, get her to listen to the better speakers. Try to sample a dvd or cd that you have and listen to it before you go to a store, take it with you too. I got my girlfriend to agree to upgrading by getting her to listen to the speakers I had selected.
I started out real cheap, $250 or so. Next step was $500 for 5.1 with a sub I still use. Then it gets interesting. I got a set of Polk Lsi15s for the fronts at a killer price of $399 for the pair. I have a Polk LsiC coming for $356.95. Rears are going to hurt though. I want the Polk LsiFX and the best price I've found is $660.91. I'll probably have about $1800 in speakers. That's at the prices I paid, the Lsi15s go for anywhere from $650 to $900 each normally.
I really liked my Hsu set-up but just got the bug to go bigger and better. Something you might want to take a look at:
For $1099 plus shipping:
http://www.hsuresearch.com/products/enthusiast2.html