Looking for hw suggestions for high concurrency OLTP app

Hi all,

I'd like to get suggestions from all you out there for
a new Postgresql server that will replace an existing one.

My performance analysis shows very *low* iowaits,
and very high loads at times of peak system activity.
The average concurrent processes number is 3/4, with peaks of 10/15.
*Sustained* system load varies from 1.5 to 4, while peak load
reaches 20 and above, always with low iowait%.
I see this as a clear sign of more processors power need.

I'm aware of the context-switching storm problem, but here
the cs stays well under 50,000, so I think it's not the problem
here.

Current machine is an Acer Altos R700 (2 Xeon 2.8 Ghz, 4 Gb RAM),
similar to this one (R710):
http://www.acer.co.uk/acereuro/page9.do?sp=page4&dau34.oid=7036&UserCtxParam=0&GroupCtxParam=0&dctx1=17&CountryISOCtxParam=UK&LanguageISOCtxParam=en&ctx3=-1&ctx4=United+Kingdom&crc=334044639

So, I'm looking for advice on a mid-range OLTP server with
the following (flexible) requirements:

- 4 physical processors expandable to 8 (dual core preferred),
either Intel or AMD
- 8 Gb RAM exp. to at least 32 Gb
- Compatibility with Acer S300 External storage enclosure.
The "old" server uses that, and it is boxed with 15krpm hdds
in RAID-10, which do perfectly well their job.
The enclosure is connected via 2 x LSI Logic PCI U320 controllers
- On-site, same-day hardware support contract
- Rack mount

Machines we're evaluating currently include:
- Acer Altos R910
- Sun Fire V40Z,
- HP Integrity RX4640
- IBM eServer 460

Experiences on these machines?
Other suggestions?

Thanks.

--
Cosimo