Honestly, if you like your Maxima, which it sounds like you definitely do, I would just go back to the Nissan dealership and test drive a new Maxima, and also a couple other Nissan models just for fun. Why mess with a good thing?
Otherwise, if you're bent on a different model (which I can understand, I like to mix things up!) I would take a serious look at Volvo's, Infiniti's, and also check out the Pontiac G6. All are under 40k. If you feel really spanky, go gitchoo a Saab 9-5. It's about the size of your Maxima, but GOD ARE THEY FUN.
Seriously, I'm a huge Nissan fan, they make really well-made cars. You like the one you have, so I'd look at a new Maxima before anything else out there.