Hosting the OS and software is where the SSD really earns it’s pay. I have 2 systems built this way. That’s your choice. It won’t make a bit of difference for your preview. The problem is you’re not going to want the wasted space of a fast drive that you marked off as OS only. Have you seen any 120 GB (spinning) drives. I don’t think they make them anymore. That’s why I use SSD’s that are just big enough. (Mine is 60GB.)
Your cameras are AVCHD – processor intensive. Decent for the work you describe. Are you doing a lot of color correction. This can be expensive for processor cycles. A well timed white (or color) balance may help.
According to your post you will have great editing but slow rendering.
For that money (or less) you could put together a 2-chip Xeon, although the chips in your price-range would not be hyper-thread capable. Still 8 real cores at about ~2.5 GHz is nothing to sneeze at. Don’t let me confuse you.
Overall the price is about right.
Jeff
Windows 7 64-bit, ASUS P6X58D, i7 960 3.20GHz, 24.0GB DDR3, 12TB connected storage