I have a P5 with the TT-PSU and am using a Benz Micro Glider SL and love the combo. However, before upgrading to the Benz I was using a Rega Exact 2. I must say that for the money the Exact 2 is a good cartridge, but I think you can do better with a MC with your budget. The Benz Micro Gold would be a great cartridge and retails for about $375 dollars. I have heard the Benz gold cartridge on a Rega P3-24, and it sounded great. Plus Benz has a trade up policy if you want to move up their product line in the future.
You might also look at Stereophile and The Absolute Sounds's recommendations on cartridges in your price range. Be sure to select a cartridge that will compliment your phono stage in terms of loading and gain. I have also heard that Grado cartridges can cause a humming issue with Rega tables. So, I would ask around about about the humming issue with the Grado cartridges before you make your purchase. Good luck and happy listening!