RAID controllers for Postgresql on large setups

Inheritted a number of servers and I am starting to look into the hardware.

So far what I know from a few of the servers
Redhat servers.
15K rpm disks, 12GB to 32GB of RAM.
Adaptec 2120 SCSI controller (64MB of cache).

The servers have mostly have 12 drives in RAID 10.
We are going to redo one machine to compare  RAID 10 vs RAID 50.
Mostly to see if the perfomance is close, the space gain may be usefull.

The usage pattern is mostly large set of transactions ie bulk loads of
millions of rows, queries involving tens of millions of rows. There are
usually only a handfull of connections at once, but I have seen it go up to
10 in the few weeks I have been at the new job. The rows are not very wide.
Mostly 30 to 90 bytes. The few that will be wider will be summary tables
that will be read straight up without joins and indexed on the fields we
will be quering them. Most of the connections will all be doing bulk
reads/updates/writes.

Some of the larger tables have nearly 1 billion rows and most have tens of
millions. Most DBs are under 500GB, since they had split the data as to keep
each machine somewhat evenly balanced compared to the others.

I noticed the machine we are about to redo doesn't have a BBU.

A few questions.
Will it pay to go to a controller with higher memory for existing machines?
The one machine I am about to redo has PCI which seems to
somewhat limit our options. So far I have found another Adaptec controller,
2130SLP, that has 128MB and is also just plain PCI. I need to decide whether
to buy the BBU for the 2120 or get a new controller with more memory and a
BBU. For DBs with bulk updates/inserts is 128MB write cache even enough to
achieve reasonable rates? (ie at least 5K inserts/sec)

A broader question
For large setups (ie 500GB+ per server) does it make sense to try to get a
controller in a machine or do SANs have better throughput even if at a much
higher cost?

For future machines I plan to look into controllers with at least 512MB,
which likely will be PCI-X/PCI-e.. not seen anything with large caches for
PCI. Also the machines in question have SCSI drives, not SAS. I believe the
most recent machine has SAS, but the others may be 15K rpm scsi

Whether a SAN or just an external enclosure is 12disk enough to substain 5K
inserts/updates per second on rows in the 30 to 90bytes territory? At
5K/second inserting/updating 100 Million records would take 5.5 hours. That
is fairly reasonable if we can achieve. Faster would be better, but it
depends on what it would cost to achieve.