Oh, gawd. Lossy to lossy. You're gonna kill your sound. Re-rip your CDs; re-download your digital. Rip and download in lossless if you can afford the space (and, really, it's more affordable than ever; a 1TB HD can hold around 3,000 CDs in lossless ALAC (Apple lossless) or FLAC (iTunes and iPods won't do FLAC, however; don't know about Macs in general). If you need even smaller files later, you can make MP3s or AACs from the lossless files.
If you really must convert any WMAs to another lossy format, I suggest AAC instead of MP3. Better sound and smaller files at the same resolution, and nearly everything accepts AACs nowadays; anything that doesn't must be really old. And only convert to the highest resolution.