Freezing problems are often RAM related. Which exact RAM kit do you have? Have you manually set the RAM speed, timings, and voltage to their rated specs in the BIOS? Have you run Memtest86+ overnight to test for RAM errors? That's where I'd start. I doubt it's related to your 5850 or Windows 7.
The problem could also be related to your HEC PSU. If these problems just started when you got your 5850 it's possible the PSU can't put out enough clean power under load to properly power the 5850.