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So what about the cheaper USB interfaces, like from RME. Will they improve anything regarding latency or dropouts over my old M-Audio?
Seems like nothing from RME is exactly cheap.
An RME USB interface is very likely to give latency results that in the real world are indistinuishable from a PCI card. Unless you can tell the difference between 3 and 4 milliseconds or are recording a very high audio track count at the same time while monitoring using the DAW's input echo function. And the incresed latency that result from that is down to the computer's ability to handle the data, not the interface or its driver.
RME ASIO/Core Audio drivers are amongst the best in the business. No, RME are not cheap, in the same way SSL or UAD aren't cheap. If you want inexpensive try a basic Focusrite or, if all you need is stereo audio output not input, try using on-board audio chip on the motherboard using Sonar's WASAPI driver settings. That may do what you need.
You may even find that not using plugins that generate a lot of additional latency until the tracking has been done and you're at the mixing stage may solve your latency problem and cost you nothing. Some plugins add a lot of latency because they need time to do their stuff and plugin makers often don't tell you how much. One that does is Waves, and while some of their plugs add zero latency at some samplig rates other plugins might add as much as three seconds.
Managing that kind of thing is part of running a DAW. The only way to avoid having to deal with the issue is to use hardware processors and synths, not software, at least until everything is tracked and latency no longer a problem anyway.
All this, of course, may or may not apply to a Ryzen system. They're too new for there to be much data about, though Jim R's opinion is worth paying attention to, he does this stuff for a living and has for quite a time.