Overhaul or upgrade Tektro Novela brakes?

Hey guys,

The bike I've got at the moment (Fuji Nevada 3.0) has Tektro Novela brakes. They are pretty poor, I can't lock the back wheel no matter how hard I try and on a moderately steep slope front+back brake won't bring the bike to a stop. When pulling hard on the levers there is a spongy feel, as if the cables are stretching.

I'm just getting into the game so I don't want to spend more on this bike than I have to but I would really like some better performance from my brakes, more as a safety thing than anything else. If I over-extend myself (fairly likely since I'm learning) then I'd like to know I can stop.

I plan to replace the cables and was considering replacing the pads as well. I read somewhere that a light sanding of the discs with 400-800 grit can bring them back up a bit too.

Do you think just replacing cables/pads and sanding the discs would make a noticeable difference or should I consider upgrading the whole system?

Cheers,
Dan.