AM3 CPU in AM2+ Mobo = significant bottleneck or no?

An AM3 Phenom II X4 will run the same in an AM2+ board as it does an AM3 board, the only difference is the memory you can use. There is a very minimal performance difference in DDR2 and DDR3, you'll loose at most 5% performance.

i current run a
AM3 Phenom II X3 TripleCore 720 3.9GHZ cpu in a
GA-MA770-US3 am2+motherboard
with 12 gig DDR2-1066
so the answer to your question is no it wont bottleneck it at all as long as the ram u pick is (DDR2-800) and above that is good
cause the ddr2-667 and 533 seem to bottleneck it a bit
in benchmarking i ran the
DDR2-533
DDr2-667
DDr2-800 id say this is min for AM3 chip set before u notice performance difference
DDr2-1066 between this and the 533-667 was a massive improvement in speed +benchmark stats so yea
DDr3-1333
DDR3-1600 -not much noticeable difference between the 2 and the ddr2-1066

so if picking ram go with thee DDR2-800 or 1066 if u don't want to spend alot 1066 for max performance for $$

so hope this helps

well, i don't think that the motherboard itself is more expensive, i think the ram is and its not that much, btw you should get DDR3 board, the memory is getting cheaper and DDR2 is getting more expensive and in the future if you want to upgrade your ram you might be SOL because of price (look at DDR prices compared to DDR2 and DDR3)

Not much of a bottleneck - but go AM3

It's only a matter of time before AMD drops the DDR2 memory controllers in AM3 CPUs - dang nice of them to make the effort but they won't do it forever.

And we don't want to hear you in a year complain about that new AM3 CPU that won't work in your AM2+ mobo - LOL

Do AM3 CPUs work in all AM2+ motherboards?

Most AM2+ motherboards have had BIOS updates released to allow the use of AM3 processors, but to be sure you'll need to check your motherboards cpu support list which can be found on the manufacturers website. You also need to consider that a high number of entry level AM2+ boards only support 95W processors, so once again check to see what processors are supported as 125W processors may damage your board.