downloaded music can be better than cd's provided a. it is not recoreded from a cd but from a source better than a cd like sacd, audio dvd, or a digital master copy of some kind, maybe from vinal provided everything from the needle to the drive motor to the amplification to the dac are all high quality and in proper working order b. it is not recorded with speaker and mic, or throught other analog means where it is rare to have hifi equipment these days. and c. that it is recorded in a lossless codec or in a standard codec but at a higher sample rate and bit rate than cd's.
mp3's are ok at 320kbps
so if you want to try mediafire or you use bit torrents look for mp3 320kbps or flac or 24bit @96khz but be careful if you dont want to be sued into bankruptcy not that it is a high risk but it happens enough to make it into the news occasionally.
so if you want a good repeatable and not prone to wear listening experience get a good hifi dac/preamp there may be good internal cards but it seems all the hifi guys like dedicated external usb versions prolly for emi insulation but im sure if you find an internal card with a comparable dac it will be fine, get a hifi power amp with good sheilded cables (short cables are better for less capacitance, cat-5 is also good if you want to make your own interconnects with low capacitance though george l's cable with brass rca style ends would logically be a great choice for hifi the cable is cheap maybe comparable to cat-5 as long as you buy bulk cable without the special goerge l's ends) and get some quality speakers. vinyl is great for occasional listening but it don't last forever.