We used to have a south-facing porch and found the same. It was either roasting or freezing. I found a "tower" of spider plants did the best.

One solution would be to put up a few roll up blinds - those bamboo ones are quite good and cheap. If you can possibly leave door and windows open on hot days it helps a lot. In winter have a fan heater on a frostat. I have not lost anything doing it this way, but as I'm at home all day its easy to keep an eye on it. When I go out I lock the door from the house into the conservatory and leave the outside conservatory door open.

With the frostat cacti will do well and pelargoniums will continue to flower sparsely on and off all winter.

I would suggest some sort of blinds too (even just if it's on the south aspect) and a heater for the winter. Our conservatory was built with no heating and was perishingly cold over the first winter. We bought a small convector type heater with a thermostat and timer on it just to take the chill off the room in the winter (otherswise we would never have set foot in there between Nov and Feb).
I have a lemon that is happy in the conservatory all year but I have to move some things into the house during the peak summer months - my bonsai gets miserable and loses its leaves in the summer if left out there.

We also used a small office fan to keep it cooler

I have a real show stopper for you - Erythrina Crista Galli.
You must provide plenty of humidity and water, but mine survived the winter in my garage ok. I brought it back into the conservatory a month ago, cut it back to six inches and it has now put on three feet of new growth.
Flickr photos at the link below........

http://www.flickr.com/search/?q=erythrina%20crista%20galli#page=0