OK, serious suggestions now that I know your budget. As I said the other day, I seriously would not bother with parcel shelf speakers or rear door speakers. If you have $500 to spend, I would look at the following speakers, in this order:
Focal 2-Way 6.5″ Component Speakers | Westside Sound & Image (rrp$479)
The new flax speakers from Focal are beautiful. Absolutely awesome value for money and can rival speakers in the $700-$800 range if powered right and tuned (your amp and lack of features on the Sony head unit will be a limiting factor but they would still be a huge step up in sound as long as they're amplified).
Focal 6.5″ 160W Component Speakers | Westside Sound & Image (rrp$449)
30th Anniversary release P165's, also from Focal. Have seen them on special for $399 before but I'm not sure they're still being made. If they're available, they're an absolute steal for the money.
Morel 250W 6.5″ 2-Way Speakers | Westside Sound & Image (rrp$449)
Morel Tempo 6" - Morel are outstanding value for money. More bassy than Focal speakers from what I have heard of them.
Alpine DD Linear 6.5″ 160W Component Speakers | Westside Sound & Image (rrp$399 - on special $309)
I haven't heard these speakers but Alpine's DDLinear range has always been good. Their DDLinear range used to be upwards of $800+ so while they're not what they used to be, they're certainly a much better choice than Alpine's lower level offerings and Pioneer speakers, etc.
Focal 6.5″ 2-Way 160W Component Speakers | Westside Sound & Image (rrp$399)
The standard version of the anniversay edtion mentioned above, still sound great. All Focal speakers sound great to be honest.
Hertz 6.5″ 2-Way 250W Component Speakers | Westside Sound & Image (rrp$399)
Hertz are good value for money. These are a touch more bassy than the Focals, but don't have the same clarity. I have a friend with these in their car and to me they sound a bit muddy compared to Focal speakers. These are still a better option than Alpine Type-R/Type-S though, to my ears.
If you really want to go budget because you need some of that $500 for installation and dynamat then these are a bargain at $250 - Morel Maximo 6
Morel 6.5
As you would have noticed, these are all from Westside Sound and Image in Cannington. If you go there and ask to speak to Fabian and tell him that Craig from Perth-WRX sent you, he'll look after you. His installer Jeremy has done a couple of things for me that I didn't have the tools to do and he was very thorough.
Whatever speakers you choose, you should dynamat the doors and get MDF spacers made and this will ensure you get the best out of your speakers (amplification aside). Whether that is part of the $500 budget you've stated or on top of will be for you to decide once you speak to the guys at the stores you visit. Maybe start by asking them how much they will charge for installing splits and dynamatting the doors, then let that pricing dictate the rest of your budget.
Everyone's ears are different and some people just cannot tell the difference between $200 speakers and $1000 speakers. Listen to as many speakers in store on their demo boards as you can. Obviously a sound board doesn't replicate how they will perform in a car door, however it does let you compare apples with apples under the same conditions (ie. all on a flat board, amplified by the same amplifier). Your ears will quickly tell you what speakers they prefer listening to.
Focal speakers are a bit light on in terms of bass, but the clarity of them leave most other offerings at the same price level for dead. Their tweeters tend to be quite harsh, but most of the Focal crossovers have the ability to attenuate (decrease output of) the tweeters by 2,4 or 6 decibels from memory. I used to run my old Focal K2 speakers at -2db. Focal actually design and make their own speakers... they're not re-branded items like most of the audio industry offers.