Question: What is the optimal RAID1+RAID5 configuration and why?

My thinking is:

RAID 1 SAS 15k 146GB drives for the OS.
RAID 5 SATA 7.2k 300GB drives for the data.

My opinion is that the OS partition should be as fast as possible, and that the data partition should have a compromise between high storage space and low cost as there will be several HDDs to purchase initially and possibly more if I need to expand the volume.

EDIT--
This is going to be an SBS server for a small business so blistering data throughput isn't as important as cost and efficient utilisation of the HDDs.

It will be running SBS 2011 with at least 15 concurrently connected AD users (SBS allows up to 75 connected users). 

There will be a single exchange mailbox store with a 2GB mailbox per user. MSSQL 2008 will be running with at least 1 medium sized database (20 GB). 

Sharepoint will be accessed concurrently by at least 5 users. There will also be file storage requirements: one home folder per AD user and I would estimate that each user would store at least 1GB of data in their home folder. 

Also a couple of network shares accessed by no more than 2 users at a time. 

Finally the server will be used for DHCP, DNS and WSUS.

EDIT--
Oracle is irrelevant. It will be storing files, and some databases under MSSQL.

Answer: 
Why should the OS partition be fast? Chances are everything that you need from it will be loaded in memory most of the time. The only thing that should be hitting the disk is the pagefile if you leave it on the OS volume. Seems like a waste of money to me. 

As for the data volume, we can't answer that for you. You need to size your workload and get an idea of IOPS and throughput. After that it's just simple math to get the best disk configuration. If it's just a few users sharing files on it, then 7.2K drives should be fine for both arrays.

These are, of course, just guesses since you didn't provide any meaningful metrics for us to base a recommendation off of.
It depends on what you want to put in the data partition, Do you want to use the data storage partition for a database ?

The best and fastest solution should be a RAID 1 + 0, unless you are planning to use the data partition with a database server, in that case a RAID 5 could be better.

If you want OS partition as fast as possible with 2 disk, do a RAID 0 (and plan a disaster recovery on a network drive for example), if you can buy other 2 disk you can do a RAID 10 also for the OS .