bottom end:
pistons<high compression for NA, low comp for FI>
rods
sleeved<FI>
balanced rotating assembly
when it comes to block work the only thing that really is going to alter your power is what u do with the pistons. that and obviously a fresher motor is usually stronger.
top end:
port and polish <different levels, different prices, different gains>
cams
cam gears
upgraded valvetrain<retainers, springs, valves>
different cams have different grinds as to change what rpm ranges they make the most power in, gears are basically only useful in quickly adjusting timing(great for tuning), port and polish jobs allow for better flow and generally do increase your power output. an upgraded valvetrain in conjunction with upgraded rods will safely allow u to increase ur rev limit, and also obviously improve your power.
if you are trying to decide what to do next with your car first decide if you plan on going FI or NA, if you want to go FI and really want some major boost do your block work, dont worry about the top end till later