I would sacrifice the little bit of extra HDD space and RAM for a better processor. You'll get better results NOW and you can always add more RAM and HDD's (external's are great for files that you don't use often)
Definitely get the PC though, even though Mac's are fantastic, like chemist said you can use premiere for your video editing and it works just as well as final cut. The transition should be painless and then you won't have to learn new music software.
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