In this situation, you might find that your guitar recordings improve in quality, but the synth recordings won’t. This is because the synth recordings are already at optimal quality because they are created by your computer perfectly. The only thing you might notice is you can hear them better because the soundcard outputs better quality to your headphones/monitors. Also, if the soundcard is good you will have less latency, which can be extremely useful when recording and listening back at the same time.
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The guitar recordings could improve in quality because the quality of your external soundcard will be better than your internal one. You might find there’ll be less noise due to the external device being better shielded/higher quality.
As for whether the possible improvement in quality justifies spending $200 on a new soundcard is difficult to say – really it’s just personal preference. If the external soundcard provides you with extra inputs/outputs for other devices which you might use in the future, it might be a good investment. However, if you’re already satisfied with the quality you’re getting now, you might be better off using that money on something else.