Most music that you purchase from the iTunes store is either 192 or 256kbps Mp3 from what I've understand.
I wouldn't buy music from the iTunes store. It's overpriced, and the quality is not the best.
What you should start doing is buying full album CDs, from a place like Amazon, or your local store, and staring ripping them to a loss-less format (ALAC, FLAC, WAV etc)
For you, since you use iTunes already, I would recommend ripping them to ALAC (Apple Lossless Audio Codec) which would have the file format .m4a.
I wouldn't recommend ripping any songs to FLAC (Free Lossless Audio Codec or .flac) because it's incompatible with many Windows and Mac music players including iTunes.
The newly ripped ALAC (.m4a) files are 100% natively compatible with iTunes and Apple devices/software. Just import them into your iTunes library and there you go!
Note that loss less audio files do take up significantly more space on your hard drive. An average 2:00-4:00 Mp3 file may be around 3-10MB in size, while an equivalent Losless audio file might be 10-40MB in size.