While Sagara's writeup above is fairly comprehensive, a note about the 600MHz RM7000A might be in order.
The most noteworthy thing about the 600MHz RM7000A, is that SGI never shipped it. But wait, I hear you exclaim, I've seen an hinv from an O2 with a 600MHz RM7000! Yes, indeed you probably have, but this was never factory equipment. It was a user-performed modification. Some years ago, it's not clear exactly when, an enterprising hacker realized that there wasn't much difference between the RM5200/300, and the newly available RM7000A/600, and that it would be possible, with just a few purple wires, to replace the CPU on an RM5200 module with the faster part. So he tried, and lo, it worked. Not only that, but it was stable.
Testing shows that the RM7000A/600MHz is slightly faster for integer math than the R12000/400, though it lags behind a bit in floating point performance. The other side to the tradeoff, though, is that the R12000 module is wide and blocks the second hard disk slot, while the R7000 module is single-wide and allows use of both SCSI bays.
Pre-modified RM5200 modules and entire upgraded O2s sometimes go up for sale on Nekochan or eBay, and now and then, hackers on Nekochan modify another run of them, generally on a bring-your-own-RM5200 basis. Because of this, the RM5200 modules (and O2s that are thus equipped) often sell for higher prices than their performance would justify. Because of this, if you're a collector who doesn't care about the 600MHz modification, avoid the RM5200/300, and get one of the R10000 modules or the RM7000A/350 instead.
Work is ongoing on a method to retrofit an RM7000A/900 or RM7000A/1600 to one of these modules, but all efforts so far have foundered on issues with the boot PROM, for which source is not available and reverse-engineering is phenomenally difficult.