Buying your girlfriend the cheapo bike won't help. I've seen several couples with the man on a flashy dually while the woman rides a cheap ass hardtail. I can only figure they don't actually want their female friend riding with them. The ladies usually don't look happy. Try renting her a bike to see if she is interested in riding. Or let her buy her own bike. Maybe give her the three bills and have her come up with the rest. $300 bikes are all the same saving the frame geometry. She also might want to preride whatever bike first, to ensure a good fit which will go a long way towards getting ANY newbie hooked on the "sport".
As for you, the same consumer rules apply to bikes. You get what you pay for. Most bikes in the $600 range are again fairly similar excepting frame design. Go ride some bikes. One last thing about the discs. Buying discs on a bike with an MSRP near to your price range will ensure low quality brakes and lessor quality components than a non disc equipped machine. On the upswing it will give you a *****un' look, you'll already have the disc wheels and parts are cheap on th internet.