Sorry to hear you're having problems!
Random lockups/reboots can be the result of a lot of different things. When you're having consistant lockups and reboots you're usually (but not always) looking at one of two things: heat problems or RAM problems.
Since you think you've ruled out heat problems, I would bet you're having a RAM issue. Since you're using an Asus mainboard and OCZ RAM, you're probably not having a compatibility issue (although, who knows?). One thing that's setting off a red-flag for me is that you're using 4 modules. When you use 4 modules with an AMD64 system you're probably going to have to raise your CAS latency to 3 (or 2.5 at lowest). Also, most mobo manufacturers will recommend lowering your memory clockspeed to 333 DDR (166) instead of 400 DDR (200). Check the packaging or OCZ's web site for suggested settings for your RAM in a 4-module configuration and try those settings first. Most AMD systems run best with 2 modules instead of 4.
There is also the possibility that one of your modules is/has gone bad. To test this (and also to test your settings/timings for stability) the industry standard application is Memtest86. Download it here:
http://www.memtest86.com/
Create a bootable floppy and run the full test. If you're seeing more than a couple of errors then you've found the problem. Make sure you try the test with the manufacturer's recommended conservative memory settings before you assume that one of the modules is bad. Once you've determined that one is bad start the process of elimination to figure out which one (or pair) it is. Then it's RMA time.
If you determine that you're not having RAM problems it's time to start checking your Windows error logs to see if there's an entry indicating what caused the crash. However, I'm betting it's a RAM issue. Let me know what you find out. Good luck!