If it's not too late, I'd suggest getting a new machine spec'd for a database server, meaning faster drives and GB NICs. This may be within the same price range of your first box but it should give you a big performance boost instead of just scaling your websites in which you will still be exposed with MySQL bottlenecks from your first box's hardware configuration.

Also, if you have not done so, High Performance MySQL book from O'Reilly is a good read.

As always, goodluck!