Most good audio players have plugins available for FLAC; also, you can download FLAC-WAV and FLAC-MP3 converters easily.
As for the potential differences between MP3, it depends a lot on what content is there and how it was mixed and encoded. Anything in mono, speech, a lot of classical and jazz, and country can get away with lower bitrates than rock and metal. The main difference comes down to how well you can hear high frequencies (and of course how detailed your system can be) - I can 'reliably' hear a lot of theft-protection systems, CRT whine, fluorescent lights, and other such things that a lot of people can't hear, and I can tell the difference between anything with a low-pass as extreme as the average MP3 encoder. You're not hearing it through iPod plugs, or desktop gaming speakers, but if your hearing is all there (and I know that's a big 'if' given that this is a metal forum) you can tell MP3 from WAV through a pair of PX100s up to 160kbps without a second guess if you use something like TGE as reference. At 320kbps it's very hard to hear the 'normal' content but to the best of my knowledge that high content is still lost. I've personally been tested for hearing quite a bit above the norm, and I pulled off that same double-blind test in a prior challenge over SHN and OGG, but given the extent to which I see 128kbps MP3s I'd assume that I'm not exactly normal.
Oh, and try Fraunhofer's MP3 encoder, if you can - smokes LAME, no question. Fraunhofer developed the technology, and are still doing it better than anyone else that I've heard about.
Jeff