Don't rush out and buy a bunch of discs... find a friend who's been playing longer (or make a new friend), and take a stack of discs to a field and see what works for you. But don't just base it on what goes the farthest--base it on what goes well and is controllable.
You can also look at the bags of various players and see what top players throw. Basically, it's usually 5 molds for 90%+ of all shots... a putter, a mid (you've got both of those), an overstable driver for wind, skip shots, forehand rollers, etc (Firebird is usually the preferred Innova version of this, Predator for Discraft... unsure about other companies), a neutral fairway driver (sidewinder can work here), and then a distance driver (something like a Destroyer).
My guess is that as you improve, you'll grow out of the Sidewinder.... they tend to not fight enough to hold up to greater armspeed. So I'd start auditioning discs for the fairway driver and overstable driver slots (don't worry about a distance driver yet).... so if you were going to buy anything, buy something like a Firebird and something like a Teebird or Thunderbird and master those. They are discs that will serve you well all the way to a World Championship if you need them to!