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Hi bikerescue
A single GPU wont give you a lot of income because with 30 mh/s if you mine eth you will earn $ 623.92 per year at the current price of eth and difficulty
I suggest you to buy more GPUs if you could because you have a big advantage not paying for power
tnx for the comment and the upvote

Is there a downside? I mean, if I make $40 this month of currency, and it goes up x10 (as many of the have), am I risking anything? Can't I just do this when I am not using it? Sorry for all the questions, I am having so much fun learning about this.

I think the only downside is the PC should be on for 24/24 7/7 so if you don't have a warranty on your GPU you might loose it (Shit Happens), otherwise if you mine a good coin it will always go up so if you hold your coins you'll be earning more money