I don't know much about the 330 or the 550, but the 320 and the 540, although they made use of x86 processors, were in fact true SGI machines.
They featured the Cobalt graphics chip and a unified memory architecture similar to that of the O2, so in a way, the 320 was the "direct descendent" of the O2.
The unified memory architecture gave these machines a huge advantage over the PC's of the day when it came to their available texture memory and this helped when it came to the manipulation of large images.
The 320 and the 540 were in-fact really cool machines and their failure rested mainly in the fact that SGI didn't deliver followup models as they had promised, and this lead to a further loss of confidence in SGI's strategy at the time.
If I have to compare my 320's performance to that of the PC of the day back in 1999, the 320 would blow the PC away.