Well…it depends, if you just need your pre-production tracks as guide tracks even MP3 in 96 kbits/s is O.K. But if you want to use some of the tracks in your later production, WAVE or AIF files are the right joice. The sample rate should have at least 48kHz/24 bit – but 44,1 kHz/ 16bit is also fine. Never! upsample your files, there is no advantage in sound quality. Better record your stuff from scratch in a higher sample rate (48kHz or 96kHz, 192kHz is overdone). We normally produce in 48kHz or 96kHz depends on the production.