Here are my thoughts. You can ignore them if you want. I think you need to define what you really want in a second machine. Do you just want to sew a straight stitch? Do you need buttonholes, hem stitches, decorative abilities?
In my opinion, when it comes to sewing machines you get what you pay for. Walmart doesn't service machines they sell so you might get something cheap up front but you pay for it somewhere in the end. I'd look for a reliable dealer. (I think warranty service makes or breaks a deal for me.)
I haven't seen a reliable Singer for a long time although some will swear by them. I saw them being sold on TV explaining how they can bring you a better emboidery because they removed the computer from the machine so their machine is actually cheaper. I had to laugh because now people have to actually connect their computer to the machine. Didn't sound like much of a deal to me. I don't want a computer in my sewing area connected to my sewing machine by a cord. AWKWARD.
I hear Brother is doing some redesigning so you might want to be careful on those.
I've heard good things about Babylocks, Berninas and Viking. I use a Pfaff (wouldn't trade it for most sewing but I would look at another machine for embroidery because I really am not fond of the software mine has).
You can pick up a very nice Janome (it won't embroider a lot of designs) for around $350 (DC2010) but is has around 50 stitches.
Maybe a really good trade-in/second hand machine is the answer to the delimma? There are some awesome deals out there.