1. I highly recommend you get an AM3 board. LGA775 is dead.
2. Yes, do go with DDR3, there is really no point in getting DDR2 any more.
3. If you already have the E6500, sell it and grab a Athlon II X3/X4 and OC that.
4. A Good AM3 board will only cost you a little more but it will be more future proof (as in you can put in a X6,etc).
5. I personally would go with the Gigabyte board.
What is the difference between, lets say an athlon processor and a phenom processor? Is it that the phenom has a L3 cache?