PhenomII x3 is AM3, works with AM2/+? Confused.

there are two 'versions' of the Phenom II. One for Am2+ boards (and some upgraded AM2) which is intended as an upgrade for exsisting AM2/AM2+ users with AM2/AM2+ boards with DDR2 ram.
The 720 is a tri-core cpu (X3), and is AM3 (the new AMD platform) with DDR3 RAM. You just need to make sure you get the right CPU, for the right board.

The 720 is a a verya ttractive purchase cpu btw.

yes, the AM3 cpu's have BOTH ddr2 and ddr3 memory controllers. Therefor all new cpus should run in old boards, with BIOS update of course. Old AM2 socket cpus won't run in AM3 motherboards cause they lack the ddr3 memory controller.
THE reason i love AMD, sorry, but it's the easy upgradability. Guess what my next upgrade will be?

Not quite right. The 720 (or any AM3 cpu) which is AM3 works in AM2+ boards with ddr2 as well as AM3 boards with ddr3, making these processors fly off the shelf like the last bag of dorito's in Texas.

am2/+ has 941 pins, am3 has 938 pins.

They both have a ddr2 memory controller at DDR2-1066 but only the AM3 cpu's also have a ddr3 controller rated at DDR3-1333(I7 is 1066)

If you want to future proof and upgrade later then get an am3 based pc, if you don't then get a ddr2 based pc since 1333mhz ddr3 is only slightly better than 1066mhz ddr2 due to latency's.