A couple of basic questions that might help with this:

How big are the databases? How much RAM will you have in the machines?

The more data you can store in memory via buffers and caches, the better off you'll be. Generally speaking, more RAM is better, especially as your database is read-heavy. As much as possible, you want to avoid reading from disk. When you do wind up reading from disk, the 15K disks will make a difference, though I don't have a feel for exactly how much more performant the 15K disks will be.

I don't know how well the Perc5 cards scale to multiple disks, but spreading the i/o load over more spindles is a good thing. Spreading load over multiple striped disks improves overall performance linearly. 2 disks can provide double the performance of a single disk, or in a mirrored config, 4 disks can double the performance of 2 disks. If your storage needs are light, you're better off to have more smaller disks than fewer larger ones.

2.5" v. 3.5" disks are functionally equivalent assuming spindle speed, cache, and back-end interfaces are identical. If you're looking at buying 15K 3.5" SCSI drives vs. 10K 2.5" SAS or SATA drives, the equation changes dramatically, however.

Can you post any relevant stats for your database? The output of a mysqlreport (http://hackmysql.com/mysqlreport), for instance, the current performance specs of your existing db, etc.

We're starting with 8GB of ram possibly upgrading to 16GB if needed. The DB is very small now (less than 100MB), and baring ridiculous growth over the next 6 months, i dont foresee it becoming much larger than a few gigabytes in that period.

There's really no existing performance numbers because hte site is not yet live yet. Im mostly trying to get a gauge of whether its commonly accepted that getting 10K drives is very sufficient in high performance environments with quad-core procs, or whether 15K is a generally much wiser move (even if it means 4x15K disks as opposed to 6-8x10K disks).

Thanks.

With 8G RAM and a couple of gigs of data, you can keep pretty much the whole data set in memory AND have lots of room left over for query caches and buffers. In fact, it sounds as though you may be overengineering this whole thing.

The performance of 10K drives is very good, but is not as good as an equivalent 15K drive. This much should be obvious. Which direction to go on that I think depends on the disk accesses within the database, and it sounds as though you'll be able to stay in RAM for the vast majority of your work UNLESS you wind up having to do a lot of joins that require on-disk temporary tables to be created.

That said, I'd rather have 8x10K disks than 4x15K disks in order to spread load across spindles AND to allow me to keep data and logs on completely separate spindles. You should get better overall performance out of 8x10 than 4x15. If the alternative is 7200RPM disks, the story changes entirely. I'm sorry - I don't know anything about Dell's 2.5" disk offerings these days.

I agree with mwallace, if your database is smaller than RAM basically all data will be cached in RAM and the speed of the disks will basically be negligible.

Basically the only critical thing that will be written to disks in these cases is if you run InnoDB and you want to keep the transaction log to be flushed to disk after each transaction (default setting for compliance).
But since you have the battery backed up PERC5 controller which will cache the writes for you, you don't need the faster disks for this either.

In my opinion there is nothing wrong with choosing slower disks on your server if you know that the bottleneck is never going to be the disks. Which is the case with very small DB's.
That way you can invest more in CPU's instead if that is your limitation.