As far as I now you don't need drivers, on Linux. You just hook up your keyboard to your pc. Then you start LMMS, and go to the midi tab of an instrument. You should be able to select your piano there. Click on the small piano keyboard icon, then you see a list and your Yamaha keyboard should be the...
With making music you cant have too much ram.
If possible I would go for 8 - 16 gig.
Not sure if LMMS will run much faster, but you definitely can use more heavy plugins.
For lmms the graphic card is not that important, for Blender it can be, but graphics cards are expensive at the moment.