SecondSpin is a good option (and fast!) but the way i always converted my mods was thru DigiBoosterPro. This way i could edit the mods a bit. A bit of panning to create some "real" stereo mods. Just render it as AIFF/WAV, load it in SecondSpin and convert it as Mp3.
To make it sound a bit more realistic and professional: Load the AIFF/WAV in the nifty program "Cathedral" and add a small itsy-bitsy reverb. This'll sound great!!!
HAve phun!
