I've not used Sonar before, only Cubase/Pro Tools.

A decent sound card is essential and if you want vocals/acoustic guitar etc. a decent condensor mic is too (I bought a Seinhesser 2/3 years ago for £100 and it was ace). A decent compressor is almost essential (v. cheap second hand) - I've not seen software that can efficiently replicate the hardware. I should add that I've not actively done anything in the last couple of years so this may not be an issue anymore.