new h/u and new speaker

Hey guys, I installed a viper 350 plus thanks to some of you guys. Now I have a Pioneeer DD 4200 on the way with 4 Infinity Reference component speakers. I have a 2000 Toyota Celica GT-S that has the 8 speaker setup with factory amp. Supposedly the amp sends around 25 watts x 4. The H/U is 50 watts x 4. Do I need to by-pass the amp and install all new wire, can I run the factory amp with the new H/U? Will this cause loss of power? Do I need any special harnesses or converters to do any of this? I ordered all the adapters and harnesses recommended by crutchfield online. Should I get a Alpine 300 watt 4 channel amp and run to the 4 speakers? If so, what harness would I need if any to install this amp with the H/U? A amp install kit?   What gauge wire would suffice for running to speakers and what gauge wire for H/U to Amp? Sorry I know it's a lot.

Metra makes a 70-8113 amp integration harness. It uses the RCA outputs of the radio to connect to the factory amp. Its the route to go if you want to utilize the factory amp. If not you would need an 8 gauge amp kit for the 300W amp. You could then extend the wires from the factory amp to the new one.

Your new radio may say it is 50X4, the most power you can get out of a 12 volt supply into a 4 ohm load is 17.5 watts per channel.  Get the 8113 mentioned above and call it a day.

Car amplifiers have DC to DC power supplies.  This steps the voltage from 12VDC to plus and minus 20volts in a small amp, and plus and minus 100 volts in some larger amps.  No radio manufacturers put DC to DC supplies to run the amplifier section.  Some do have DC to DC supplies to power the op-amps.

Thanks for the reply! I ordered this http://m.crutchfield.com/mt/www.crutchfield.com/p_120708113/Toyota-In-dash-Stereo-Wiring-Harness.html?tp=2977 I'm assuming this will work just fine? My last question will be do you guys think it would be worth buying the 300 watt amp, and are the speaker wires long enough for me to pull out the factory amp and just use the speaker wires running to them?