Depending on the roofing color and orientation, having even insulated & sealed ducts in the attic can add a full ton of peak cooling load. If the ducts are not sealed on every seam & joint it can add more.
Putting masonite over the registers very likely reduced the amount of stack-effect infiltration through the house by making the ceiling more air-tight, which has a dramatic effect on reducing latent-load, if somewhat lesser (but still significant) effect on the sensible load. Caulking/foaming/blocking every ceiling penetration of plumbing & electrical, flue-chases etc. is worth it, as is air-sealing the foundation sill & band joist, putting in
better dryer-vent backflow prevention etc. in the basement to slow down stack-effect infiltration, which makes a difference both winter & summer.
Some states and utilities offer rebate type incentives for installing mini-splits that meet minimum efficiency standards (eg:
http://www.duke-energy.com/indiana/savings/smart-saver.asp. and it's highly likely that you qualify for some of that. But it's also possible for reasonably handy people to do a DIY install (but I'd still have a qualified tech complete the full commissioning & testing to ensure efficiency), and you can get a pretty-good 1 to 1.5 ton ultra high SEER heating & cooling mini-split for under $1500 at internet prices:
http://www.amazon.com/20-25-Mitsubi...f=sr_1_12?s=hi&ie=UTF8&qid=1338473532&sr=1-12 or
http://www.amazon.com/Fujitsu-Single-Split-System-AC12RLFW/dp/B007TY27UG/ref=sr_1_1?s=hi&ie=UTF8&qid=1338473927&sr=1-1. The difference in cost between a cooling-only or heating & cooling version is usually a few hundred, and if you heat with oil, propane, or electricity it'll take a HUGE chunk out of your heating bill too. (Not so much if you're heating with gas, unless you have cheaper than average electric rates.)
Installed by a pro it'd probably run you close to $3-3.5K for a 3/4ton or 1-ton, and a 1.5 ton would typically be in the neighborhood of $4K. A 1600' house with a reasonably high-R attic, decent windows and reasonable air tightness wouldn't usually need more than a 1.5 ton even at 110F, and the
1% design temps for IN run about 90F, so a 1-ton in the largest zone would likely carry the whole thing without much trouble. (If you've been living with a window-shaker in the bedroom and it's mostly keeping up, it'll be a noticeable improvement in comfort, noise, and operating cost.)
If your east & (particularly) west windows are clear glass (even double-panes) it's worth buying
low-E storm windows (at $150-200/window, for a typical double-hung), which will cut the solar gain when the sun is low in the sky, since overhangs can't protect it on the E & W sides the way it can on the S side. This makes for big uptick in wintertime performance too. If the windows you have are low-E gas-filled U0.34 or lower don't bother, but a low-E storm over a bottom-of-the-line U0.60 double pane beats code-min replacement window performance at a fraction of the installed cost. South facing clear glass still reflects most of the summer heat due to the high sun angle, but in winter when the sun is lower at mid-day the solar gain is useful rather than harmful. If single-pane clear storm windows on the south side are worth it though.