*vonvergelenriqu: maybe you should describe muna your current setup.

ano ang car mo?
ano ang current HU?
ano ang current speakers?

you said meron ka ng amp at Targa speakers.. nakakabit na ba ito or hindi pa?

ano ang amp model? ilan watts? ilan channels?
ano klase at ano size ang Targa speakers mo at ano plano mo gawin dito?

to improve your sound system, you really need to start with the basic, i.e. good HU and good separates, then add amp and additional speakers or sub. you can refer to the photos na pinost ko sa taas.

capacitor
cross over
tweeter
1 set subs na 8inch or mas better kapag 12"
separates
wiring kits
bakit need mo ng capacitor?
bakit need mo ng hiwa-hiwalay na crossover, tweeter, at separates? kapag bumili ka ng separates, 1 set na yan (includes woofer, tweeter at crossover). or am i missing something here?
12" sub maybe better than 8" depende sa car mo at sa gusto mong bass. madali mag-add ng sub so kahit later mo pa bilhin yan.