A 66MHz, 64-bit PCI bus can handle 4 gbps, so unless it's a badly-engineered bus, that shouldn't be your problem.

Also, do NOT remove the autonegotiate feature from your network cards and switches if you're running gigE over copper.

1. If your links are not running at 1000 mbps, full-duplex, there's a problem. Papering over that underlying problem by forcing the link to 1000 mpbs full-duplex doesn't fix the problem.

2. Disabling autonegotiation on copper gigE places you outside the specifications of IEEE 802.3: (http://www.sun.com/blueprints/0704/817-7526.pdf)

I'd look to be sure you do all the tuning that Oracle advises for running over NFS. Especially make sure you're using jumbo frames.

And you might very well need to look into replacing your old hardware and software. There have been a lot of hardware and software advances in networking performance since Solaris 9 was current.