i need a new system! please help!

hey guys, i jus joined!

first of all, i have an 85 nissan 720 king cab, it has the 4in, speakers in kick panels, and thats it...
so i bought a new pioneer head unit rated at 50x4, and new speakers for the kicks, they are 5 1/4" pioneer , and i have an amp already, it is an orion 800extreme... my oldest brother bought it in like 99 or 2000, and never used it! so he sold it to me for $150, and i got a 10" 600 watt pioneer sub in a sealed box...  the system bumps sretty good... but not good enough for me!

so i have about 400-500 bux to spend on upgedes... so i was thinking about adding 6 1/2" pioneer speakers in the doors, and a set of 6x9's in the back where the flip out seats were... and another sub...

so right now, the 2 speakers are fight off the head unit, and the sub is off the amp... so when i put in 4 othew speakers, plus the extra sub, what will i run them off? this amp only has spots for 2 subs and thats it... and the head unit only has 4 spots for 4 speakers... but i will have 6 speakers..... where can i plug them into? and do i need to get a cheap amp to run just the speakers off of? and the subs just off the orion?  i need help!  

i will also need power cap, my lights start to flicker a little when it on at 10.... and when ever bass hits, i can watch my amperage gauge go from like 12 to almost nothing!  i was sitting at a stop light today, and the base hit and stayed, and my gauge ddroped, and the truck died out! then it almost wouldnt start cuz the battery was so low!