I've been doing some research in this area over the last week and haven't finished yet, but I'll shared what I've found out. Info if probably over the top, but I've been spending alot of time because I want the machine to perform to peak for the money spent.
OVERCLOCKING
You can overclock the 920 easily with the Mobo you've selected and is good value for money. You'll be able to get 3.4-3.8 easily but you'll need:
* an aftermarket heat synch / cooler.(without getting complicated with a liquid cooled system, I like the Thermalright 120 or the Skythe Mugen 2 air cooling solutions (but you need to double check if these will fit on your chosen Mobo with the cards you plan to install as they are quite large)
* a case with excellent cooling design and tidy cabling
* if you search the internet for overclocking 920 for your board you'll find enough info to get this done. Here's an excellent article on the Core i7 architecture with a good introduction to overclocking on the platform.
VIDEO CARD
Although the 4850 was recommended to me, feedback I've received so far from other forums (I haven't confirmed this to be true) is that Premiere will not utilize the 2nd GPU in a dual GPU / graphics configuration (e.g. with either CrossireX configuration with two separate video cards or single card with dual GPU's, 2nd GPU is not utilized), so your second 4850 you've included may not be giving any added performance there (unless you're wanting to use for games). I'm considering the GTS 280 as i also like to play games occasionally.
STORAGE
Minimally you need separate physical storage drives for your OS/applications and your media/video.
For OS/application drive:
You could do perfect with a smaller cheaper version of the drives you've already selected (say 300GB). For me, I'm going for the instant access times when loading apps, etc. with a 128GB solid state drive from Corsair for about $325,
http://www.newegg.com/Product/Product.aspx?Item=N82E16820233087&nm_mc=OTC-Froogle&cm_mmc=OTC-Froogle-_-Solid+State+Disk-_-Corsair+Memory++Inc.-_-20233087
but am also considering an interesting option included with Mobo ASUS P6T Deluxe - an onboard SAS RAID controller (SAS ports look like SATA ports, but you can put 2 x 10,000 (test result showed 137 MB/S Read Avg) or 15,000 RPM (showed 172 MB/S) SAS drvies in RAID 0. It gives almost 3 times the storage for same price as solid state drive, but also I hear these drives are quite loud, so I’m not sure yet.
Here is some more info on the latter option:
http://www.legitreviews.com/article/910/1/
http://www.newegg.com/Product/ProductReview.aspx?Item=N82E16822116058
For your video storage:
The hard drives you mentioned are fine for standard definition video. However, if you want to edit high definition or otherwise have complex / advanced ideas for your projects, you'll need something with higher read rates than just those single drives. This gets pricey, so depending on what you're intending to do, may not be appropriate for you. Options I've investigated for this so far are:
* Native SATA RAID solution included with Mobo configured for RAID 0 with 2-4 drives, although onboard RAID solutions vary in performance and some provide almost no improvement vs a single drive. this requires investigation for your particular motherboard (or you could just buy the number and size of drives you want, then if performance is not suitable, add card from next option and use the drives you already bought)
* PCI Express RAID controller card (about $300 will get a decent card for this purpose) configured for RAID 0 with internal SATA ports to connect to 3-5 SATA II hard drives. For performance, the RAID controller has to have substantial cache memory and chip, will usually indicate DDR, etc. in the description (e.g. http://www.newegg.com/Product/Product.aspx?Item=N82E16816116042)
Above options require a computer case with plenty of space for lots of hard drives and good cooling design.
* You can also go with an external SATA drive RAID enclosure with eSATA connector, but these tend to be more expensive, but its easier to expand capacity later, replace drives that fail, etc.. Some of these even support SAS drives now. Some have a RAID controller card built in the enclosure, so all you need is to connect it to your computer via eSATA cable. Others come with a PCI Express RAID controller card with eSATA ports on it or require you to boy one t
Note that RAID 0 provides the best performance but also increases random chance of volume failure (if one of the drives fails), so you'll need seperate backup of your video files. Alternatively, RAID 5 will allow recovery of a failed disc by simply replacing it, but performance will not be as good in this configuration (whether you notice a difference in performance btwn RAID 5 & RAID 0 depends on your chosen solution).