Yes, the GTX460 is serious overkill for an HTPC (media only) system. That doesn't mean you cannot use it. If it's within budget, I'd probably recommend keeping the GTX460 as a spare for your gaming system, should the unthinkable happen, and adding in a low-end graphics card (like the HD6450 or GT620/GT630).
Your system build looks fine except you'll never need more than 4GB of RAM. As far as a retail Blu-Ray drive, I found this Samsung Blu-Ray Drive
at Amazon, but you might want to look to see if there are any sales going for blu-ray software. Sometimes you can find it for as much as 50% off. If there are any sales, just get the OEM from Newegg.