Please Help A Newbie Options

[slimjimmed]

I have a new G5 and a usb midi keyboard i recieved as a gift. Since I am new to the field of computer music, i need some basic advice. I want to know what kind of software i need to buy if i want to have a collection of musical voices to play with my keyboard. What i am really looking for is weird tribal and spacy music that i could use to score my short films. Additionally, it would be cool to have a nice piano and organs.
I'm looking for cheap($100-ish or below) software. I don't know if it would be a major contradiction to spend so little money and get quality sounds since i am ignorant to using my computer for recording?
I don't know if i need Vst's, synths, samples, etc... I am frickin' lost. I was looking at Sonic Refills and ProSessions, and they seemed to be the type of sounds i was digging, but i don't know if i'd have tgo shell out for some proprietary software that costs$$ in order for them to work with my keyboard. Thanks for your patience.