Switching to Oracle will be painful, kind of like switching from a bicycle to a Maserati; a lot of your SQL will have to change, and you'll have to hire a DBA.Better to switch to two MySQL instances, one for writing and one for reading, with basic replication to keep the read database current. This is well-documented.
If you think you're going to go well beyond the load capacity of MySQL (what that means for your app depends on the quality of your schema and your SQL), look into postgreSQL first; it's a low-admin solution, a step or two up from MySQL, not 8 steps up like Oracle with respect to administration requirements.
But you can do some basic calculations now, starting with calculating how many queries for each page, broken down by read queries and write queries. Look in your logs to see which pages get how many hits (by percentage, e.g. page x gets 14% of the hits, page y gets 5.5%, etc.), and you'll have a decent usage model. Next time each of the write queries and each of the complex read queries and you'll be pretty close to having a nice spreadsheet for estimating the capacity of your application on its current hardware.
To see what difference hardware can make, just time the queries again on the new hardware and plug the numbers into your spreadsheet.
It's obviously a little more complex than this, but for the first step in capacity planning, this is sufficient (in my experience).
--Kurt
