Check out this thread for lots of tips & links to more info: How to find music

How to use Bitzi

And these threads for info about which kinds of files to avoid (not so much for the virus factor since you're using a Mac but just to avoid crappy files that won't play): Virus thread: Typical virus spam file sizes - Make a note of them (click on blue link to see)

Fake files showing up in search results

Generally, a file with a higher bitrate will be of higher quality. A higher bitrate means a bigger file. I'd avoid files with a bitrate of less than 192, unless it's a really rare file & you can only find it with a lower bitrate.

It's always worth checking files with Bitzi & previewing during download - sometimes a file can show with a high bitrate but sound terrible