If my fiance and I go business casual style-dressed up, (her in a dress, myself in a normal black suit w/o a tie), is that too much? It would be a dinner on a Sunday night, and not really wanting to overdress/stand out....

I think you're fine at SunDial business casual. The hotel frequently has conventions and corporate guests, so you should blend in perfectly and not feel over dressed. Cheers!

My cousin and I went for her birthday brunch and she was totally dressed up! I don't see anything wrong with going business casual.

When I first started going it was more business than casual. You should do great. They have live jazz on Wednesday and Thursday. (Fyi) it starts around 7 or 8.