So first, I am correct in thinking that AM3 chips work in AM2+ correct? They are setup to handle DDR2 or 3?
From there, I was wondering, at what point is an upper-tier chip overkill with diminishing performance gains since the board and memory are a previous generation.
Basically I'd love to throw a new 6-core into my AM2+ setup to update it, but I am thinking that I may not see significant differences from purchasing a lower priced quad core since they aren't truly meant to run with DDR2 in the first place.
For one AM2+ boards are limited to DDR2 RAM which provides lower bandwidth than they would get an a DDR3 motherboard. Even so, this can be partially compensated with low latency DDR2 RAM thus reducing the difference. Generally, an AM3 CPU will operate with a lower HT Link Speed in an AM2+ board, but that's the last thing to affect performance and can be manually set anyway ^_^. The difference is small though and won't hugely affect performance assuming you're using some decent DDR2 1066 RAM in dual channel mode.
and AM3 socket cpu will fit in an AM2+ socket motherboard. you just have to make sure that the motherboard has the right BIOS which will support the AM3 cpu's. as for performance, i believe its like a 5% to 10% performance loss in some cpus. not really sure though
Unless you use programs that specifically make use of more than 4 cores you will not see much of a benefit over going with a quad core of similar speed. It's true that the X6 CPUs have turbo core, but a 955BE can be easily tuned to run at 3.7GHz and above assuming a decent cooler.