AMD DDR2 Limitation

Ok basically i have 4gb of ddr2 ram but as many of you will be aware there is a limitation which currently applies to am3 cpus in ddr2 (or am2+ mobos) where you cannot have more than 4gb running in dual channel mode (am i correct?) so basically if u want more than 4gb it will all run at 800mhz.
So as i say currently i have 4gb of OCZ Blade ram at 1066mhz in my asus m4a78 pro mobo (which is am2+), and i want to know if its worth buying a new am3 mobo and ddr3 ram so i can have more than 4gb, or whether i should just buy 800mhz ram to go with my 1066 (as they will be downclocked anyway).

So basically to questions: Is ddr3 really that much faster and is it worth me buying a new am3 mobo and memory to use more than 4gb ram, or should i just buy 800mhz ddr2 to add to what ive got.
Also im a bit confused when it comes to this limitation in the first place, so if anyone could clarify on it thatd b good too. Thanks.

amd phenom II x4 965 @ 3.4ghz
OCZ Blade 4gb (2x2gb) @1066mhz
ASUS M4a78 pro
WD Caviar Black 500gb
Antec Earthwatts 650w psu