The 5830 is easily faster than the 4850. Should definitely see some performance gains, depending on the rest of your system.
As rolli59 pointed out, can you identify the brand and model of your Power Supply? It may also help to know what resolution you're playing at, and what CPU you have just to make sure there's enough to handle the GPU in the first place.
my CPU is E8400 running at 3.77 Ghz, 8 GB DDR2 RAM and my MotherB is gb P43. HD is Caviar Blue 640, My power supply is GigaByte too, 450 Watt. I play at the highest resolution in my monitor 1920 x 1080
I'm getting anew 600 Watt OCZ PSU tomorrow or after.
Since you are OCing your CPU, I assume you plan to OC your GPU. That being tha case, the 450W PSU is not enough in the long term. Right now, you are on the low end of acceptability. Adding a quality PSU capable of handling more will help keep the overall system stable longer.
OCing places additional stress (primarily heat) on components and your system (like a chain) is only as strong as its weakest link. In your case, your current PSU is weak. Get the new one. HOOAH!!!