Let's start with the case. You do not have enough room for a larger case, so the question is how to fit in as many disks as you can.
Have a look here http://www.newegg.com/Product/Product.aspx?Item=N82E16817121405&cm_re= supermicro_sata-_-17-121-405-_-Product to help you create more disk space. The downside is that your video card can not be too long, so you may have to rearrange some components. An alternative is not to use 3.5" drives but the smaller 2.5" models, at the cost of storage space, but then you can use http://www.webconnexxion.com/raid/product_info.php?cPath=63&products_i d=652 to create more space for disks. You can consider this cage for all 4 Velociraptors you currently use. The Velociraptors are in fact 2.5" disks but mounted in a 3.5" cooling body. I don't know if that will give sufficient cooling however.
Next memory: For the time being you have sufficient memory and with current prices it does not look interesting to invest in 4 GB sticks. Wait and see how CS5 will perform in the future would be my advise.
Disk setup: You make backups of your C drive to an external with Acronis. So, nothing to do about that. It is good.
Then you have 3 VR's that can easily be used in either raid0 or raid5 from the motherboard on the ICHR10 chip. I would prefer raid5 despite the load on the CPU for security reasons and accept the lesser performance. In addition, I think that disk performance will not be a limiting factor if you do not overclock your 920.
Then your media disks. Before I can give you a definite answer, can you give me an indication of how many disks you can fit in or how many 5.25" bays you have available. Consider too, that it may be worthwhile to remove any fan/temperature displays that occupy a 5.25" bay.
Raid controller: that depends on the number of disks you can fit in or that you may want to attach in the future with an external drive cage, even though your space is limited. So please answer my previous question first and I'll follow up on it.
updating my earlier answer i complete becouse your qustion of graphic-card space.
Having checked my Zalman MS 1000 case again it will be bossible to put in modular Front HDD bay Nr 3 + van, without having problems with Saphhire 4870.
total 3 modular Front HDD bays (+van for each), each front bay 3x hotswap dives 5.25 are possible. Total 9 drives 5.25.
see attachments (BTW, the second shown comp is my wifes...conneced by home network)
Great news because that gives you a lot of options. With one BR burner installed you have 9 5.25" bays left.
One is to install 3 SuperMicro cages (you don't need to install them all at the same time, but nice for the future) to add up to 15 3.25" disks in hot swappable bays. Or you can choose to use two SuperMicro cages for 10 3.25" disks and add 3 Addonics cages for an additional 12 2.5" disks.
Lots of choices here and you are not really limited by your case. You may be in terms of cooling however.
C: Velociraptor for OS & Programs
D: 3 x Velociraptor in raid 5 on the mobo for pagefile, media cache, previews like you intended
E: initially 4 or 5 disks (1 or 2 TB, up to you) in raid 3 for media
Raid controller: Areca ARC-1680 iX-12 with 2 GB ECC cache and BBM.
I suggest the 12 version over the 8 version because it has the possibility to expand cache memory from the default 256 MB up to 4 GB, although one wonders if anything more than 2 GB is worthwhile, given the price difference between the 2 and 4 G sticks.
With this configuration you have the possibility to add a multitude of disks to expand your storage and increase the speed of your raid if you wish.
So all you need is two SuperMicro cages to start with, the disks and the Areca controller with cache and BBM.
Hope this helps.