It depends on the CPU. Some Sparc 5's were faster than than the 20's. And if the 20 is a Hyper-sparc - Run!

As far as the OS: If you're planning on running Solaris with CDE I wouldn't go past 2.5. The performance hit just isn't worth trying to run anything better (it will run mind you but s-l-o-w.) I have an Ultra 1 running Linux and I love it.