SAN vs Internal Disks

Hi,

We are currently running our DB on a DualCore, Dual Proc 3.Ghz Xeon, 8GB
RAM, 4x SAS 146 GB 15K RPM on RAID 5.

The current data size is about 50GB, but we want to purchase the hardware to
scale to about 1TB as we think our business will need to support that much
soon.
- Currently we have a 80% read and 20% write perecntages.
- Currently with this configuration the Database is showing signs of
over-loading.
- Auto-vaccum, etc run on this database, vaccum full runs nightly.
- Currently CPU loads are about 20%, memory utilization is full (but this is
also due to linux caching disk blocks) and IO waits are frequent.
- We have a load of about 400 queries per second

Now we are considering to purchase our own servers and in the process are
facing the usual dilemmas. First I'll list out what machine we have decided
to use:
2x Quad Xeon 2.4 Ghz (4-way only 2 populated right now)
32 GB RAM
OS Only storage - 2x SCSI 146 GB 15k RPM on RAID-1
(Data Storage mentioned below)

We have already decided to split our database into 3 machines on the basis
on disjoint sets of data. So we will be purchasing three of these boxes.

HELP 1: Does something look wrong with above configuration, I know there
will be small differences b/w opetron/xeon. But do you think there is
something against going for 2.4Ghz Quad Xeons (clovertown i think)?

HELP 2: The main confusion is with regards to Data Storage. We have the
option of going for:

A: IBM N-3700 SAN Box, having 12x FC 300GB disks, Partitioned into 3 disks
into RAID-4 for WAL/backup, and 9 disks on RAID-DP for data, 2 hot spare. We
are also considering similar solution from EMC - CX310C.

B: Go for Internal of DAS based storage. Here for each server we should be
able to have: 2x disks on RAID-1 for logs, 6x disks on RAID-10 for
tablespace1 and 6x disks on RAID-10 for tablespace2. Or maybe 12x disks on
RAID-10 single table-space.

What do I think? Well..
SAN wins on manageability, replication (say to a DR site), backup, etc...
DAS wins on cost

But for a moment keeping these aside, i wanted to discuss, purely on
performance side which one is a winner? It feels like internal-disks will
perform better, but need to understand a rough magnitude of difference in
performance to see if its worth loosing the manageability features.

Also if we choose to go with DAS, what would be the best tool to do async
replication to DR site and maybe even as a extra plus a second read-only DB
server to distribute select loads.

Regards,
Azad