I think it would be most worthwhile to save up and invest in that audio interface you don't have. If they come with 1/8" cables it'll probably be as good as XLR to 1/8" so there's no need to spend money on adapters you don't need, but the best option for the long term would be to buy something that's intended to work with XLR and will offload a large burden of work from your computer.

No problem, and there is! Sound cards and audio interfaces use things called Digital to Analog Converters to turn the data that is being sent by a computer into an electrical signal to drive the speaker cones in your monitors. The better your DACs, the higher quality the signal sent to your monitors. If you're using your motherboard's onboard audio card, you're asking it to do a LOT of work that it was not designed to do - this will lead to a loss of audio quality and overall computer performance.