2 HDDs? depends, if you're planning to do a lot of recording, yes.
If not, then a 1TB one should be enough.
If i were you, i'd sacrifice a little bit more on the processor side and a little less on the Video Card side, but since you're not a hardcore gamer and you want to record/edit your Minecraft videos, then i'd suggest this:
Go with the Phenom II X4 955 and stick with the HD 6770. Because video editing/encoding needs a lot of processor power. If you stick with the Athlon II X4 640 you'll have less power to encode/edit videos.
You could add a 2nd hard drive only if necessary, if you really think that you'll need it.
Cheers