I would put everything on the RAID 6. And I'd go on record as not liking the weird RAID setup.
The RAID 0 pair has no redundancy. I wouldn't trust it in a production environment regardless of whether it has SSD or old-school disks.
I hear one or two scary stories involving sudden, early drive death about SSD every month, which just makes it that much more worrying. Often, it is consumer-level kit. I'm sure that a few reading this have had great experiences. But...if it is a revenue-generating site or a public-facing site, it just isn't worth my job. Another thing is that a SSDs are probably totally overkill, considering that the RAID 6 probably can't read as fast as the SSDs can write.
If the pair of SSDs were a RAID1, that would be different. I'd stick the log files (for all of the databases) on there, and maybe tempdb itself.
Since you can't change the RAID setup, I don't see that you have any choice.
But it's not really that bad, if you have all of that RAM to yourself.
Assuming that you aren't running any other heavy software on that box, since you have 2x 9 GB databases and quite a bit more memory than that, all of your data will (eventually) be cached in RAM. Once the data is in memory and assuming that you don't do lots of large writes to tempdb (which an OLTP database generally shouldn't do), the speed of your drives isn't as critical. The big exceptions being log writes and log backups (which read the log files), if your site databases are running in FULL recovery mode. The RAID6 is going to be (relatively) slow for writes and I'm guessing that the 73 GB drives aren't 15KRPM disks, but that volume should be able to handle a few MB/second. The rate at which you need to write to the log files is determined by how busy your site is and how efficient at writes it is.
The system databases (master, model and msdb) are not large and should get hardly an I/O, compared to your user databases. It won't significantly matter if they are also on the RAID6.
I might consider using the RAID0 pair to hold backups, assuming that they get copied to somewhere else in reasonably short order, but that SSD RAID pair seems like a real waste of money.
When you do get the system up, make sure to monitor the load on the storage subsystem to make sure that it isn't over stressed. If it is, you may have to change the storage configuration to make your site perform better.