A 2006 or newer S60 would certainly be a nice addition to your long line of Volvo's. The model's a bit outdated, they've essentially been making the same car since 2001, and I believe a new re-design is in order for next year, but they're still really nice cars.

Here's 2 by me for sale:
http://raleigh.craigslist.org/ctd/1424218727.html
http://raleigh.craigslist.org/ctd/1428145301.html

That said, Volvo's are not good on gas, but they are safe, powerful, and [generally] reliable cars, but they have their quirks. If you want straight-up reliability, buy a Honda or Toyota. My parents have owned 3 Hondas ('88 Accord (which began to fall apart after I owned it), 2003 Acura MDX, 2007 Accord), and they have not had a SINGLE problem AT ALL with the latter 2 in the time that we have owned them. The Accord is great on gas, surprisingly spunky and quiet for a 4-cylinder, and great in-town or out on the highway. My Volvo is more fun to drive though, and for the MOST part has been pretty much maintenance-free for the 2 1/2 years I've had it. Nonetheless, shop around! If you're going to make a huge investment like that, don't limit yourself to one particular brand. I recently test drove a brand-new VW Jetta 2.0T and absolutely loved it, which would also be within your $20K limit.