Hearing the difference now isn't the reason to encode to FLAC. FLAC uses lossless compression, while MP3 is 'lossy'. What this means is that for each year the MP3 sits on your hard drive, it will lose roughly 12kbps, assuming you have SATA - it's about 15kbps on IDE, but only 7kbps on SCSI, due to rotational velocidensity. You don't want to know how much worse it is on CD-ROM or other optical media.

I started collecting MP3s in about 2001, and if I try to play any of the tracks I downloaded back then, even the stuff I grabbed at 320kbps, they just sound like crap. The bass is terrible, the midrange…well don’t get me started. Some of those albums have degraded down to 32 or even 16kbps. FLAC rips from the same period still sound great, even if they weren’t stored correctly, in a cool, dry place. Seriously, stick to FLAC, you may not be able to hear the difference now, but in a year or two, you’ll be glad you did.