I used to live in the Houston area (Kingwood), and we also replaced our A/C while we lived there. Here are a few thoughts and questions:
1) Is your upstairs smaller than your downstairs? It seems like the upstairs should have the larger A/C unit, since it has the heat gain from the attic.
2) Our attic only had a few (maybe 6?) inches of fiberglass, and no radiant barrier. Before replacing the A/C, take care of those things. They're much, much cheaper than the A/C. They'll also decrease the load on the A/C, extending its life and reducing your electric bills.
3) If you have single-paned windows, get solar screens or window film to reduce heat load. We hung roll-down shades under the eaves on our southwest-facing wall.
4) Find out exactly what part of your A/C isn't working. The two main parts (evaporator coil, compressor/condenser) can each go bad without affecting the other. If there's a leak, make sure your HVAC company identifies where the leak is before you agree to replace anything. As long as both parts are sized appropriately and take the same type of refrigerant, you DON'T have to replace both at the same time.