Please Help, Recording set-up Options

[Kurz1]

I have a Kurzweil K2500x, a Mackie 1202 Mixer, Yamaha Power Amp and Alesis Monitor 2's. Now I need a computer for audio recording. 95% of the time everything will be done on the Kurzweil. In the future I might want to add a vocal, or guitar, or flute. My stuff is (for lack of a better term) New Age. Yanni, George Winston type stuff. Almost all instrumental. I can afford a G5 or a G4 1.25 dual and whatever interfaces or software I need (except full blown pro tools). I don't want to "over-buy". My computer will only be used for music. I want software that is not going to take me forever to figure out how to use. I have been looking at midi interfaces, soundcards, etc. trying to make sense out of all of it. I am electronically illiterate. I would sure appreciate what people would recommend. Thanks in advance for taking the time to answer my question.

Kyle