Best hardware/cost tradoff?

I'm about to buy a combined web- and database server. When (if) the site
gets sufficiently popular, we will split the database out to a separate
server.

Our budget is limited, so how should we prioritize?

* We think about buying some HP Proliant server with at least 4GB ram
and at least a duo core processor. Possibly quad core. The OS will be
debian/Linux.

* Much of the database will fit in RAM so it is not *that* necessary to
prefer the more expensive SAS 10000 RPM drives to the cheaper 7500 RPM
SATA drives, is it? There will both be many read- and write queries and
a *lot* (!) of random reads.

* I think we will go for hardware-based RAID 1 with a good
battery-backed-up controller. I have read that software RAID perform
surprisingly good, but for a production site where hotplug replacement
of dead disks is required, is software RAID still worth it?

Anything else we should be aware of?

Thanks!