Best Setup? Options

[bjurusik]

Alright, I've tried to find answers to my questions in the forums, but I'm still confused. I'm new to computer recording and am unsure as what I need. Currently I have a 17" iMac and a Tascam US-428.
Basically I just want to multitrack with guitar and voice. I've never really used midi, but in the future I may add a midi keyboard. As I understand it, the midi keyboard plays sounds off your computer correct? For example you could chose a grand piano sound set and the notes are controlled by the keyboard. Now when you record a guitar or vocal, it shows up as a waveform. Does midi do the same thing? Say your drum track is midi, is it going to show up in the tracks window below the guitar track? Any suggestions as to what software would work best for me? It has to be reasonably cheap, 300 or less. I've looked at Cubase, Logic, and DP, but they all seem like way to much for what I want to do. Would I be best getting something like ProTools LE? thanks.