Having a Dell limits your options, its BTX so compatible coolers are scant. PSUs a;so have issues fitting also. Since you're essentially upgrading the whole thing, building a new rig would actually be the better choice.
You get better overall performance than the 570+Q6600, especially for CPU taxing games like the BF series. Plus you get a spare machine, one drive might be salvageable off the RAID setup.
However, how urgent is this? waiting a month or two could lead to newer parts and potentially lower prices. You can buy the HDD now, just everything else later. Besides, if you ever gather up more cash, an better CPU+GPU would be good also.
To answer your questions:
1. OEM is for "system builders", so limited to one PC. Feature wise its pretty much the same but you don't get a fancy box, just a CD in a paper sleeve and a key.
2. HDDs aren't close to even using the bandwidth SATA II offers, so SATA III HDDs are mostly marketing. While the raptors do go a bit faster than the normal 7200RPMs, you can get better performance by setting up a SSD caching setup or get a actual SSD.
3. Building a PC is easy look for a guide on the stickies, and the USB ports come with the case (with the power button :>), you can get fancy a card reader/eSATA/USB3 faceplate that sits in a 3.25 bay, but that up to you and can be done anytime. I am missing an Disk-drive, but you should be able to use the old one.