I will start by answering your second question first. The Yeti usb microphone is seen by windows (or operating system) as a sound card because the Yeti has a pre-amp and Analogue to Digital audio converter built in. An audio unit (like the studio 192 or audio box VSL44 which are both made by Presonus) is a pre-amp and A/D converter. You cannot the yeti to an audio device. Nor can you connect the Atom to an audio interface, again due to their own usb.
The breakout cable you are using is unbalanced. This is why your connection is noisy. I see the Eris has both balanced and unbalanced inputs.
Now, the most important thing - Audio unit or no? I would say yes but it sounds like you would need a microphone and cable to record as well which is an added expense. I bought my missus an "Audiobox Creation suite" made by Presonus recently. Included was an audio unit (vsl22 usb powered), a mic with stand and cable, a keyboard for about £250, from Reverb. So there are deals out there for some decent hardware on a budget. However, as I mention, it sounds like you need a little shopping list.
You see, you will also need some balanced cables to connect you audio unit to your monitors as well.
A two channel Presonus box will run you about £100 (+/-£50)
Balanced cables for connecting you monitors to you audio box will run you about £30
An sm58 dynamic microphone (built to last) about £100 (you may get free cable). There is nothing you cannot record and they have been used in the industry for years. I have had mine a decade and it was second hand when I got it. It's been drop-kicked numerous times - near indestructible!
A condenser mic can be got for about £100 (my missus uses a Presonus M7 I think which came in her kit, I use an mxl by Marshall, less than £100). However, a condenser mic needs Phantom power to work. Generally most audio units come with a phantom power switch. (If your yeti has xlr out, you might be able to skip the condenser mic, you haven't been specific about which yeti it is that you own.
So, yes get an audio unit! It will connect via USB to you system and depending on the unit can be powered by usb.
I hope that helps. I know it's a bit of an investment in extra bits. I hope you understand I can't guess your region and can't offer you your own local currency, only my own. However, you don't need to spend large amounts of kit or a budget of an L.A. producer.