Hardware upgrade for a high-traffic database

Hi All,

We're currently running Postgres 7.4.1 on FreeBSD 5.2, a Dual Xeon 2.4, 2GB
ECC, 3Ware Serial ATA RAID 5 w/ 4 disks (SLOW!!).

Our database is about 20GB on disk, we have some quite large tables - 2M
rows with TEXT fields in a sample table, accessed constantly. We average
about 4,000 - 5,000 queries per second - all from web traffic. As you can
imagine, we're quite disk limited and checkpoints can be killer.
Additionally, we see queries and connections getting serialized due to
queries that take a long time (5 sec or so) while waiting on disk access.
No fun at all.

We've tweaked everything long and hard, and at the end of the day, the disk
is killing us.

We're looking to upgrade our server - or rather, replace it as it has no
upgrade path to SCSI. I'm considering going Opteron (though right now we
don't need more CPU time), and am looking for suggestions on what an optimal
RAID configuration may look like (disks, controller, cache setting). We're
in the market to buy right now - any good vendor suggestions?

I'd appreciate any input, thanks!

Jason