Speaker upgrade recommendation for standard 2007 ES350 (non-ML)

[TLPLEXUS]

I have searched this topic here and the concensus that our stock non-ML system is very bad (underpowered HU and low quality speakers). So i would like to get some ideas from members here as to what to upgrade. I know at least one member has upgrade his speakers to Polk 571/651 and liked it.

My budget is around $500 so i think i can only upgrade speakers for now. Knowing that the stock head unit is underpowered, a 4-channel amp would also be nice to get IF it is do-able with our stock head unit (but i am not sure if that's possible). Please let me know if you have successfully installed a 4-channel amp into your stock head-unit. I will expand my budget a bit to accommodate if it is do-able.

The speaker specs are listed below (in inches).

Location Size Depth
Front Speaker 6.5 3
Front Tweeter 1 1
Rear Speaker 6.5 3
Subwoofer 8 4

I am think about getting the following:

Front (2 dash/2 doors): Alpine SPS-600C or Polk MM6501
or
Front (2 doors only): Infinity REF6022I

Back (2 doors): Alpine SPS-600 ; Alpine SPR-17C ; Polk MM651

Let me know what other choices that you like/recommend. I think both Alpine and Polk speakers are quality brands but i am not sure if my stock head unit is strong enough to drive them. They sounded pretty good in the stores but they were all hooked up to an amp.