While the stereo microphone in the C920 can be good enough for most, for those looking for an improved sound clarity or those who want to play instruments or music in the background without it distorting or sounding like it's behind a thick veil, a cheap and cost effective solution is the Blue Yeti microphone. Many podcasters in streaming (ustream, twitch, etc.) have used it and, while you won't be getting studio-grade quality from them, it is still a huge jump in quality from the built in microphone in your webcam.
Currently the Blue Yeti is priced at $109 on Amazon at the time of this writing. There is also a higher spec'd version called the Blue Yeti Pro for around $170.
While a laptop with a decent dual core mobile processor may suffice, if you want to achieve good quality streaming (higher resolution, more than 20 FPS) you will require more processing power. As stated before, streaming is a CPU-bound activity whether you are streaming to MyFreeCams, Chaturbate, Streammate, Cam4, Twitch, Ustream, or even Skype. Many people misconceive that the GPU (Graphics Processing Unit, aka your graphics card) is responsible for most of the work. The GPU is only responsible for your monitor display and, in the case of streaming games on Twitch, for rendering whatever game you have on at the time. It is the CPU that handles taking that rendered data into a stream.
Keep in mind the limitations of whatever cam site you work on as well. MyFreeCams currently has no true HD streaming quality available for the bandwidth they provide per stream. Chaturbate still currently forces broadcasters to stream with Flash within their browsers. Flash is an extremely old platform that can easily be replaced by a more efficient and optimized software like HTML5. Due to this, Flash will always serve as a bottleneck in your maximum stream quality through Chaturbate.
Furthermore, there are different versions of Flash used between the one used in Windows and the one used in MAC (and of course, Linux). Streaming through Flash within MAC operating systems is far superior in quality than the one in Windows. I currently have two desktops running Windows and one Macbook Pro. The two desktops are fitted with an i7-5960X ($1050 Eight Core 16 Thread CPU overclocked to 4.5 GHz) and an i7-4770K (quad core, 8 threads, overclocked to 4.4 GHz), and both cap out at around 25-28 FPS on resolution setting 800x600 on Chaturbate. The FPS drops dramatically to ~14-16 when I try to set it to 768p or 720p.
In contrast, my MacBook Pro has no problems streaming at 30-35 FPS whether it is 800x600, 1024x768, 1280x720, or even 1080p. The FPS remains the same throughout until 1080p where it drops to around 22 FPS, which would be a CPU issue since that Macbook Pro is only fitted with a dual-core i5 processor at only 2.6 GHz, not to mention it is a mobile processor.
Meanwhile, on Twitch I am able to stream on my desktops at 2560x1440 at over 60 FPS with no problems since Twitch allows you to use software such as OBS or XSplit to stream with.
Simply put, even a $5000 Windows-based PC cannot stream to Chaturbate as well as a $1300 MacBook Pro due to the version of Flash being used on MAC OS X being superior for streaming. If CB ever developed their own streaming software client or switched to HTML5 then this bottleneck would be fixed.
When streaming, it is your upstream speed that you should be concerned with. Typically streaming at 1080p at 30FPS should be done under a recommended 5mbits of upstream bandwidth. All downstream and upstream bandwidth is calculated in "bits", in this example, 5mbits is 5 "megabits", not to be confused with "megabytes" which would typically be written as "mbps" and not "mbit". 5mbit upstream is the same as saying your upload speed upper limit is around 625 kbps, or 625 kilobytes per second of "true" or "raw" upload speed.
Downstream and upstream refer to download speed and upload speed respectively. You can calculate your raw download and upload speeds by dividing the "mbits" by 8. So, someone with a 100mbit downstream would be capable of a theoretical maximum of 12.5 mbps (megabytes per second) of true download speed. The same goes for upstream.
Please make sure you are not using up any upstream bandwidth elsewhere (such as torrents currently uploading) while you are streaming if you have mediocre internet bandwidth and want to be sure nothing on your end messes with your streaming.