I purchased Acoustica MixCraft4 a couple weeks ago ($69)..I can't recommend it enough..Zuki, it can help you with your MP3's with compression, limiters and any other mastering tools (many are provided), It also can change pitch and/or tempo without effecting each other..easily..

It also serves as a VST wrapper for all the great VST instruments and effects..

It comes with a very decent GM wavetable ( I actually use/like some of the sounds as lead sounds..It also is down-loadable with several great VST's...

Check it out... www.acoustica.com