I was in your same position earlier this year and decided to install a FIA-approved seat (Sparco Pro 2000 Plus). I kept the stock 3-pt belts which was a good DE solution and greatly reduced the sliding around. This was a HUGE improvement.
Recently I went the next step and installed the 6-pt harness and added the Brey-Krause roll bar extension for added safety when I'm strapped in upright. The 6-pt harness combined with the racing seat goes the next step further to keep you in place so you can drive the car rather than hold yourself in place.
Like Topless, I haven't tried saving money when it comes to safety; Snell 2010 helmet and SFI-rated driving suit, shoes, and gloves. My wife and kids think that I'm worth it.
Here is a pic of the seat (prior to the harness install):
Threaded Mode
