The first thing to establish is what you're using your server for, specifically with the HD streaming...are you wanting it to transcode on the fly, or just serve the files to wherever they'll be viewed from? Transcoding would need a beefy CPU, but file serving needs next to nothing...my home server uses a dual core E2140 1.6GHz CPU, which is perfectly adequate. In short, you don't need extra processing power for file-serving...just look at the specs for the home NAS devices, they tend to have low power, low frequency CPUs. If you're looking at new CPUs, don't bother with the i3, you're spending more money than you need. The Pentium G620 will do the job fine. Alternatively, that Phenom II X3 would also work fine. The PSU, RAM and HDDs you have will get you off to a flying start.
I've not used Windows Server 2011, as I have the original WHS on my own system...however, the Windows server OS is user-friendly, certainly more so than Linux IMHO (which is more down to my lack of experience with Linux than a criticism of the software itself, however). I just want something that works and I didn't need to learn, and Windows fits that bill.
I managed to get a motherboard with 8 SATA slots, but it's hard to find. However, you can get PCIe add-in cards with SATA slots that work fine, so you don't need to pass up on a decent motherboard purely due to it having too few SATA ports. Also, you don't need SATA 3 for a home server unless you have a 10Gbps network. SATA 2 has 3Gbps transfer speeds, so if you have a 1Gbps home network, the network will bottleneck the server anyway. SATA 2 is more than enough for streaming lossless Bluray rips with no stutter. However, avoid Green HDDs...some can cause stutter due to how the drive is designed for environmental friendliness (WD, I'm looking at you...).
Lastly, for streaming, you should be aware that most motherboard onboard NICs are Realtek. There can be issues with them for streaming content and the occasional stutter; it's usually easier to go with a dedicated NIC on the server and HTPC (assuming you're using one).