Hypertable (et al.) v. Postgres, SSI v. MSI clustering, and hardware scaling

First, a few specifics on what I'm working with and my tentative plan.

• Hardware: 15 machines; each with 2x 3.0 GHz "Nocona" (mid-2004) Xeon, 4 GB DDR2, 2x 73 GB SCSI.
  Aggregately, this works out to: 30x 3.0 GHz CPU, 60 GB RAM, 2.14 TB HDD.
  .
  (There will also be some extra decently powered machines in the cluster, but this is the bulk of it.)

• Architecture: One high-powered single-system-image (SSI) cluster of five of those machines plus any extras, with the remaining ten working independently.

• Database: Hypertable, set up on the 10 remaining nodes to work over HDFS across them.

• The SSI cluster will act as the Hypertable master and run a custom application and the nginx Web server over Kerrighed/XtreemOS, while the remaining servers will run some form of Debian.

• The database is meant to scale and perform well given high levels of reading and writing. (~7000 users)

First of all, are there any glaring issues with this plan?

Now, my main questions:

1. How does the current architecture compare to a fully SSI'd setup or a fully MSI'd setup (LinuxPMI) in terms of performance and scalability?

2. Given that I'm working with some pretty old CPUs, does this hardware seem adequate, or should I invest in more machines?

3. For my use case, what are the relative pros and cons of a datastore like Hypertable v. a more traditional RDBMS like PostgreSQL? Would it make more sense to use HT exclusively or to give part of the data (say, user management) to Postgres (or even to use Postgres exclusively)?

4. Going off the last one, at this point in time how much of a risk am I taking in using Hypertable over HBase (or Cassandra)? It's worth noting that this won't need to be deployed until after the summer, around the time of Hypertable's scheduled 1.0 release.

5. I'm also planning on using Go, so what are your thoughts on go-pgsql and thrift4go?

Thanks.