I recommend that you try a tandem out by renting one. Both of you need to read this several times first: The Proper Method If she does not trust you completely, a tandem is not for you.

Here are a couple of singles that may interest you:
B&W GT - another project - Bicycle Repair Forums
Motiv Rocky Point II - Bicycle Repair Forums

There are very few current bikes that are very good, and those are expensive. In addition to our NV friend's advice: stay away from mail/internet ordering too - those bikes need set up, which will run you $100 to $200 each at a shop.

My advice: visit ALL of the bicycle shops in your area to determine which shop you want to do business with. Do NOT purchase on your first or even 2nd visit. Narrow it down to two shops, go to each and talk in great detail about what you want, your concerns, likes, dislikes etc. Talk to the mechanics. Take test rides on a couple of bikes. Go home. talk about it with your partner. All bike shop bikes are about equal quality. The brand does not make much difference, but the shop does. You will have a long term relationship with them. Bikes require maintenance.