Sizing a box for PostgreSQL

I have been doing some "unoficial" work with PostgreSQL and I may be able
to make PostgreSQL official (ie. use it for production work).

The question my boss threw at me was: what kind of box do we need?

How would I go about properly sizing a machine to run PostgreSQL?
There would be about half a dozen tables. The biggest is about 7.5 million
records on the rest of the tables are less than a million each.

I don't know if I will have to use one of the existing servers or if I
will get $$$ to buy a new one.

If I get $$$ I will try to get 4 X 15RPM on Raid 5with 1GB of memory and
1GH cpu.

If there is no money then I will properly get an old server (one possible
seems is a 500Mz cpu with 10K rpm drives and Raid).

We literally have HOURS to run these reports, but there are numerous
reports. My boss is concerned whether we would be able to run them. Right
now we use about about 10 Foxpro machines to run the reports and other
nightly cleanups.

On my favor is the fact that I would be able to create whatever index I
need to speed things up. Right now the Foxpro tables are used for
everything so they have only the most necessary indexes.