Faster drives for WAL than for data?

Hi all,

My company currently runs a number of both web-based and more
transactional projects on a PostgreSQL 7.3 server, and we're looking to
upgrade to a new machine running 8.0 to boost performance and handle
data growth in to the future.

Right now I'm looking at a Sun Fire V40z server in a fairly modest
configuration: 2 Opteron 848 (2.2ghz) CPUs, and 4GB of RAM. The V40z has
6 drive bays, and from earlier posts and the info at
http://www.powerpostgresql.com/PerfList/ it sounds like the best
configuration would be:

* 2 drives in RAID 1 for OS and WAL
* 4 drives in RAID 1+0 for data

However, using 73gb 15krpm drives, I'll be limiting myself to about
140GB of data storage, and I'm not sure if this will be enough to cover
the life of the server. If I stick with the faster drives for the WAL,
how significant a performance impact will there be if I use larger
10krpm drives for the data?

Also, if anyone could recommend a SCSI RAID card for this configuration,
or if anyone has any other suggestions, it'd be greatly appreciated.

Thanks
Leigh