I had pretty much the same problem right at the start of summer when we moved into the house. The attic was a measly r-12 insulated, and there were only two gable vents a 4 soffit vents in back (none up font!).
It was only the low 90's out and the attic was 120+! There was no air movement what so ever in the attic.
I had a company come in and do: air sealing, extra soffits, two turbine roof vents (we are near a lake so we have a slight breeze most of the time), and they did blown in insulation to bring it up to r30-35. I think it cost me only $1100 because the power company subsidized it as part of a green energy thing. They do a blower efficiency test audit before and after to make the house qualify, which accounted for about $900 in discount.
This all made a HUGE difference. I think the AC will be the next thing to go, but for now its keeping up. Its been 105+ out and the AC is able to keep the house (1450 single story) at a comfortable 77 degrees and still turns off for 10 minutes or more every hour (in other words, its not having to run constantly to maintain the temp).