Stick with the case it's nice and big to house all those components. Yes you did choose the right thermal compound, there are a few I think because theres different sizes of the product. Chipset heatsinks...you can get them if you want, since the mobo is said to run hot, maybe you should get them. Onboard sound is fine but X-Fi is better. Yes you can use OEM only once. Retail is unlimited. You don't need to RAID drives but it's usually recommended. If you want to get two identical hard drives, like two of those WD Raptors. Overall that is a very nice computer system you got there.