The question is, are you able to stream what you want without dropping frames? If you aren't dropping frames, then don't worry about it and try to think about what else you are doing on the system while you are streaming. Also, if you are using the graphical FMLE interface, you will save a few percentage points on CPU usage if you switch to command line, either standalone or the FMLE streamer built-in to Vmix(which is command line only).

Your graphics card should be fine unless you are inputting more than 4-5 HD inputs, even more than that may very well able to work fine with that GPU, I don't know for sure.

If you're still concerned about your CPU usage, upgrade the CPU, either a K i5 so you can overclock or an i7.

And lastly, if you are trying to do various things on the system while you are streaming, I might recommend trying to get a 2nd system or something. Streaming is pretty demanding and doing lots of other things is going to be problematic.

It should show dropped frames as one of the bits of info in the FMLE data readout while it is active. You know how it shows the actual bitrate of the video and audio streams in realtime. It also shows dropped frames. A dropped frame occurs when, among other reasons, the CPU is overloaded and cannot process everything and so it will start showing dropped frames. A dropped frame is simply one frame of your video that doesn't get processed and doesn't get sent. One dropped frame here or there, while concerning, isn't necessarily going to be very noticeable(especially on a web stream). But if you are getting constant dropped frames then that is definitely going to be very noticeable and you would need to deal with it.

Whatever your CPU usage, if you aren't dropping frames, then you're OK as long as you understand what could crop up. Might want to have auto updates turned off and make sure your antivirus isn't going to start doing stuff randomly, etc...

The core of your issue is the Processor. (sorry for the pun). An i5 is under-powered for the resolution you are trying to encode and stream. A new graphics card will not help, as FMLE is designed to use your CPU as is OBS. OBS uses FFMPEG at its core as does vMIX. So you could simply try the vMix implementation of FFMPEG rather than loading OBS. Next, try a lower streaming resolution. Third test. Try using another device to handle the streaming. This could be a PC with vMIx BasicHD installed, or a dedicated device like a Terradec, Averlemedia LGP or U3.

If you are going to the trouble of changing the mobo I would suggest that you build/buy a new system to become your main production PC. Look at latest generation i7 on a a z97 mobo with SSD main drive and Graphics card as per recommended vMix specs. Use your old machine to offload functions such as encoding or as a backup.