Sun Microsystems Enterprise 250 Owner's Manual page 71

PCI cards may be freely installed in any compatible PCI slots. Slots need not be filled
in any particular order. In most cases, the system's I/O performance will be
unaffected by placement of PCI cards in the slots. Heavily loaded systems, however,
will deliver better overall performance by installing high-throughput interface cards
on separate buses. Examples of high-throughput interfaces are dual-channel
UltraSCSI host adapters and ATM-622 interfaces.
Overall system availability may also be improved by installing redundant storage or
network interfaces on separate PCI buses.
Chapter 4 Hardware Configuration 53