More:
http://www.katedicey.co.uk/ ("On the selection, care and feeding of sewing machines")
http://web.archive.org/web/20071021104932/http://cet.com/~pennys/faq/smfaq.htm

I’ve recently visited dealers for most brands of machines in the US (I was compiling a listing for a sewing book in revision) and found some terrific deals on new machines with MSRP in the $500-1500 range (which will get you a machine that should last at least 20 years). Generally, machines below about $300 MSRP are not economic to do any but the most minor repairs on, and some (Europro for one) don’t have repair parts available.

Buying a machine from a dealer (as opposed to Walmart, Costco, etc.) also gets you basic machine operation classes, and sometimes even some basic sewing classes. Most of the "sewing problems" posted here in yahoo answers and elsewhere are actually "user errors" caused by not understanding how to properly thread or use a machine. A little backup from a dealer can be very helpful when you’re a newbie. You can also often negotiate a bit at a machine dealer… if you know you want specific feet that are not included with the machine, this is a good time to bring that up as a deal sweetener. Or a deal on actual sewing lessons, if the shop offers lessons beyond the usual free machine operation classes.

I can’t speak for all dealers, but the ones I was working with while revising the sewing machine chart were uniformly helpful to newbies who were in the store at the time, really working to make sure that they weren’t buying "too much machine" and get frustrated, and enough machine that they could grow a little in their sewing journey without "running out of machine". When I was shopping for my last machine (about 12 years ago), I mentioned that I was having problems with stitching through elastic… not only was I shown several machines that could handle elastics easily, I was shown a really fast way to do elastic waistbands that look really good. <g>

Of the inexpensive machines, I think Janome seems to have the sweet spot for good machines at a low price. In the $300+ class, I’d be looking at Bernina, Elna, Janome, Pfaff, Viking (in alphabetical order). Me, I drive a Viking, about equivalent to the Sapphires, though fewer stitches.
Avoid machines that stall when stitching slowly.

And please don’t buy a machine by stitches (or "stitch functions") by number per dollar spent. Once you get beyond about straight, stretch, zigzag, a couple of decent buttonholes, stretch blindhem and blindhem, you’re probably not going to use the decorative stitches very often.

Once you get things down to a few makes and models, you might want to check the machine reviews at http://www.patternreviews.com and see what other people think of that model.

Suggested reading at the library:

Carol Ahles: Fine Machine Sewing, particularly the beginning and ending chapters on how needles, thread, fabric and machine work together, and on selecting a new machine. Carol’s specialty is "heirloom sewing", so much of the book is devoted to that — but after you get your machine, sit down with the chapter on blindhemming and work through it. It will save you an amazing amount of sewing time.

John Giordano: The Sewing Machine Book. Excellent reading, especially if you’re interested in a used machine.

Gale Grigg Hazen: Owner’s Guide to Sewing Machines, Sergers and Knitting Machines — about how to properly care for your machine and sewing habits that can cause damage.
References :
50 years of sewing, much of it on straight stitch only machines.