Reliability recommendations

We are a small company looking to put together the most cost effective
solution for our production database environment. Currently in
production Postgres 8.1 is running on this machine:

Dell 2850
2 x 3.0 Ghz Xeon 800Mhz FSB 2MB Cache
4 GB DDR2 400 Mhz
2 x 73 GB 10K SCSI RAID 1 (for xlog and OS)
4 x 146 GB 10K SCSI RAID 10 (for postgres data)
Perc4ei controller

The above is a standard Dell box with nothing added or modified beyond
the options available directly through Dell. We had a bad processor last
week that effectively put us down for an entire weekend. Though it was
the web server that failed, the experience has caused us to step back
and spend time coming up with a more reliable/fail-safe solution that
can reduce downtime.

Our load won't be substantial so extreme performance and load balancing
are not huge concerns. We are looking for good performance, at a good
price, configured in the most redundant, high availability manner
possible. Availability is the biggest priority.

I sent our scenario to our sales team at Dell and they came back with
all manner of SAN, DAS, and configuration costing as much as $50k.

We have the budget to purchase 2-3 additional machines along the lines
of the one listed above. As a startup with a limited budget, what would
this list suggest as options for clustering/replication or setting our
database up well in general?