Intel will be out of top 50 soon
Opteron rules. These supercomputer require a level of reliability that no Intel chips can provide -- a FDIV bug is not an approximation scientists can tolerate. Core 2 duo may be quick on finishing SuperPI, but scientists need correctness.
Nobody wants Intel, so it hired SGI (a BKed Intel shop) build one for itself, with either Itanium or double-p3.
I look forward to upgrading my AM2 PC soon. A 2.3GHZ AMD ought to be enough to frag a 3.33GHZ Intel.
Intel mostly use single core Xeons for its own data centers. They probably know those double Pentium 3s aren't up to standards.