lol, i say they have to be "real" prims and hand-placed - this is artisanal work!
host machine memory is the limiting factor. CPU is a factor in speed, but memory is the ultimate "full-stop" limiter
the 576,000 were stable and all stayed visible but they were also phantom and set to full bright black so that no shading had to occur from the sun's changing position
at 576k i was using 6.8 gig of RAM out of 8 but also had a few programs open like Fireworks, Dreamweaver, Camtasia. if i closed those, i have no doubt 600k would have been possible - but for what purpose? (i was getting OCD disgusted with my OCD determination) =D
i feel comfortable in telling people that sim-on-a-stick can handle 50,000 prims and that's a lot of prims for one region! and that's what this was all about =)
oh, and as i wrote, this is a testimony to the awesome talent of the OpenSim developers - they are creating an amazing thing! =)