I'm not sure if you can with the freenas web-ui, but with the ZFS command line, you absolutely can. No harm in plugging both of the SSD's in and giving it a try. ZFS has multiple uses for SSD's.
For read caching, you can use what they call the L2ARC (level 2 active replacement cache) which keeps frequently accessed data on your SSD. Pretty simple.
For write caching, you can use what they call a ZIL (ZFS intent log). Every write to the disk is stored as an intent on ZFS, before it's written to the actual array. With the ZIL, you do multiple things:
- prevent ZFS from storing it's intent log on your actual array (helping to reduce fragmentation a bit. ZFS has no defrag feature, so beware)
- speed up writes to your array. Obviously, an SSD is faster than a spinning HDD. I can get about 500mbps on my array, using a couple 840s.
I would recommend splitting about 8GB to 32GB off of your SSD's. My array looks like this. It's important to have the ZIL (logs in the quote below) mirrored, because that's a critical part of ZFS. If you have an error on your array, the raid5 will save it, if you have an error on your cache, it will fall back to the spinning array. If you have an error on your log device, you're done.
NAME STATE READ WRITE CKSUM
main ONLINE 0 0 0
raidz1-0 ONLINE 0 0 0
sdd ONLINE 0 0 0
sdg ONLINE 0 0 0
sdc ONLINE 0 0 0
sdb ONLINE 0 0 0
sda ONLINE 0 0 0
logs
mirror-1 ONLINE 0 0 0
sdf2 ONLINE 0 0 0
sde2 ONLINE 0 0 0
cache
sdf3 ONLINE 0 0 0
sde3 ONLINE 0 0 0
The pro 1000 won't let you down. The other reason I like them is that no matter what OS you're using, they will (except for XP I think) have out of the box driver support, so if you decide to go with a different NAS OS later on, you'll be all set!
A UPS is more of a creature comfort than anything else. I got mine from a business I was supporting. Their batteries were no good, and they wanted me to replace the whole thing. I did that and took the old one. Replacement batterys cost me about $25x2 (two batteries) and it's been ticking for about three years. If you can find a cheap one on ebay or something, link the model number here and we can tell you if it's good. What you should do first is look at your actual draw from the wall. Get a kill-a-watt and plug it in between your server and the wall. what it shows will be the actual power draw of the computer. That will help you get a more well-suited UPS.
Media servers don't need backups. You're spot on there.
Let me know if you've got any further questions. I'm here all day folks!