Get a large CPU cooler (ninja or something along those lines), a bunch of fans from the recommended list and that will be a good start.
I'm not 100% sure on this, but I thought I read somewhere that the P193 case had some compatibility problems with full-size tower heatsinks, due to the size of the size panel fan possibly interfering with full height (approx 160mm) tower heatsinks.
Given this, a P18
3 case might be more appropriate, which also avoids the possibly loud side fan in the P193.
Other than this and replacing the stock fans with fans from SPCR's Recommended Fans list and considering stretching your budget to an SSD as mentioned by other posters, you could also try using RivaTuner to control the fan speeds on your GTX275 so that it ramps up past a certain temperature (a few degrees above what your temp would be when you starting gaming or doing intensive 3D work that needs GPU power).
Thermalright also make some good aftermarket coolers for Nvidia cards - their HR-03 or Trad ranges should have something that is compatible with a GTX275.