Well, first trial over. I really did expect some trouble after installing the second 512 but it booted fine and did some work with no problem. Then I ran Prime95 for 6 hours with no problem. I still need to run benchmarks as well as running P95 for at least 12 hours. Plus memtest. Maybe I can OC better with this?
Oddly enough, the system (Win2003 Enterprise Server) complained about not having enough virtual memory shortly after starting P95 and it increased it! I checked using task manager and it was using 100% of the gig. So much for not needing virtual memory if you have a gig - but this is a non-typical case.
I wasn't right at the edge of stability at 212*10 but was still happy that no problems resulted. Now, just for fun, I'll see how far I can push it.