Some gen4 Camrys came with a 6-speaker system. Some came with 4-speakers. The main difference is that the 6-speaker system has tweeters where the base of where the side mirrors are located.
The main door speakers are 6.5"- 6.75" (either size wll fit)
Crutchfield.com will show you everything that fits. I bought from them in the past and they will include any necessary adaptors for speakers & wiring harness for the deck.
http://www.crutchfield.com
If you have a 4-speaker system (without tweeters), then I would recommend getting tweeters up front.
As far as the deck,
- Do you have an iPod you want to connect to your car stereo?
- Do you want Bluetooth so your cellphone can be paired to your stereo?
- Do you want satellite radio?
These are some of the things you want to consider selecting a deck to narrow down your choices.
As far as brands, major brands such as Alpine, Sony, Kenwood, JVC, etc... all make good decks. I would suggest going to a retail store and try them out. You make like the control logic on one brand over another, buttons, etc...
A new deck and speakers will make your car sound pretty good. If you want it to sound really good, then invest in some amps, woofers, etc..but then it's $$$$.
Do think about how much you want to spend.