OK, Cat, I think I've gotten these straight:
For $100: Swap out the motherboard with your 785GTM-E45 and buy
this, but before that, please perform a BIOS upgrade if available (if not, you're good to go). No better gaming performance, though. Your current motherboard is stuck on AM2, unfortunately, and this won't give you enough efficiency for real gaming. Also, forget about Blu-Ray playback - not without considerable CPU load.
For $200: AMD A10-5800K + a good motherboard should give you 3x raw CPU performance and the ability to play almost every PC game under the sun, including several high-profile console ports at 720p, medium settings at 30 FPS or higher minimum. If the motherboard you've chosen comes with HDMI-out, sell the 210. If not, look elsewhere, as there's no method of HDMI pass-through. DVI-to-HDMI are not ideal unless you've got another method to plug into your audio system. UEFI is good for forward-compatibility and boot speed. This would take up around 100 W or 130 W.
You'll most likely want to overclock the memory to squeeze out extra performance - like any GPU, the APU needs all the memory speed it can get, especially for GPU-intensive loads.
See this for a comparison between the A10 and a comparable Core i3. Keep in mind that this is CPU-influenced only; if you use their respective built-in GPUs, the 7660D will thrash the HD 2500 like no tomorrow.
If you are really serious about video encoding, though, you'll want to wait a bit more, and try to get yourself at least $300 of budget leeway. That way you'd be able to splurge on a quad-core Ivy Bridge Core i5 (around 4x your current configuration) (get one without HD Graphics to save $10, but you WILL have to have a GPU), a good motherboard, and sell the 210 to give yourself a real graphics card that works, and be still under 155.5W for the CPU and the GPU.
If there wasn't any need for it to be in a slim-line case and within 300 W, I'd probably have mailed you my 9800 GTX+