......

you can use anything from a 94-99 Bonneville in your car, you can also use the Radios out of any Grand AM, Sunfire etc

there are many aftermarket radios which have both. unless i'm mistaken they will fit just fine. might need a fit kit. they will net you better sq, better warranty, reliability, mp3, ipod, sat radio, etc.
for example:
http://www.woofersetc.com/index.cfm?...roduct_ID=4048

Features:

* Power Output: 50 Watts x 4 Max.(18W x 4 RMS)
* 24-bit Resolution 1-bit DAC
* Ready for Ipod
* Sat Radio Ready
* MP3/WMA, CD/CD-R/CD-RW Compatible with ID3/WMA Tag
* 3-Band Parametric iEQ
* Gold-Plated Front and Rear Line Output Terminals
* Gold-Plated Front AUX Input
* Rotary Encoder Multi-Control
* Dolby B NR
* Dot-Matrix Multi-Color Display with Contrast Control
* Full Logic Deck Mechanism
* Also Available in Silver - KW-XC400
* Using KS-PD100 Ipod Adapter

KWXC410 - JVC KW XC410 CD/Cassette Receiver Combo with MP3/WMA and SAT Radio Ready

not the best by any means but it'* decent enough...

personally i wouldn't use a factory radio unless it was the only option or for price concerns.

Don't "factory" head units have the same power output: 50 Watts x 4 Max.(20W x 4 RMS) as all of the aftermarket head units?

no all factory units do not.
yours possibly i don't know. who mentioned power anyways?
but power output isn't the main reason for upgrading to a unit as i posted above...
MP3/WMA and SAT Radio Ready an EQ etc...