I was running a temporary 6 6.5" setup to try, but unless you are already pushing max watts on the original 4 and need to add more, you're better off having the original 4 with more power for clarity,volume. I tested with 1 4 channel amp(75w/ch. , and one 5 channel amp( so thats 9 channels 1 for front tweets (whoa!- 75w on those puppies!), 1 for front 6.5", 1 for rear 6.5s, 1 for auxiliary 6.5" in the back door, one for subs. I was able to bridge one 4to2 channel amp to the front for 150w/ch., and another bridged 4to2 channel for the rear at 200w/ch. Plenty good, but the 5th sub channel was bought for 2 10", not that powerful running 2 12". eventually going to run 1 12" from it, and get another mono amp to run one 8". Should be a truly full range setup at that point. two tweets front, two 6.5" front, two 2-way 6.5" rear, one 8", one 12" that may move into rear door. Im just trying to use up all my junk lol.

My recommendation would be speakers now, sit on a nice unpowered, deadhead deck, maybe 1 5channel amp, 1 sub. something that incorporates sub controls or at least eq from deck, and put in all together later. Good sound, not expensive, stealth, and good quality albeit not earth-shaking- just high quality. This way youll be putting maximum watts per speaker to yield really good sound from them. Its better than having lots of speakers for volume, but putting mediocre power to them.