Difference between am2 am2+ and am3 socket

Check the Gigabyte website. They list all CPUs that will run on each specific model mobo under the "Supported CPU List". You may need a BIOS update however.

Gigabyte.com and look up the cpu support list for your motherboard.

There is no such thing as "8 gigs of sli dual channle ddr2-6400". SLI has to do with video cards, it has absolutely no relation to memory.

The latest chipsets are AM3+.

Am2 sockets take Am2 chips,
Am2+ sockets take Am2 and some Am3 chips (bios update may be required as mentioned)
Am3 sockets take Am3 chips
Am3+ (are you seeing a pattern here?) takes Am3 and some of the new zambesi chips (bios etc),
for specific pinouts check Google, wiki or similar will have pictures of each layout if you need to cross your eyes some  
and also as mentioned, check your motherboard manufacturers site for a cpu support list, it will tell you what you can put in there and if it requires a bios update or not to accept that chip
Moto
Moto

AM2 and AM2+ are DDR2 only while AM3 and AM3+ are DDR3 only. AM3 cpu-s can be used in a AM2/+ board but will only use DDR2. A AM3/+ can not use a AM2 only cpu. Phenom 2 can be used in any of these board provided that the bios supports the cpu however all previous generations are not compatible with AM3/+. AM3+ is for Bulldozer and BD can be used in a AM3 board if bios supports but some features might not work.

What video card would you use with AM2+?

Hmm... Any!  

The type of video card is not determined by the CPU socket, but by the addon-card Slot types. (Like the PCI, AGP, AGP x4, AGP x8, PCI-E 1x, PCI-E x8, PCI-E 16x)

It is easy :
PCI Cards only go into PCI Slots.
AGP Cards only go into AGP Slots.
PCI-E Cards only go into PCI-E Slots.

But :
AGP x1 and x2 cards can go into a x4 slot, but without benefits.
AGP x4 cards can go into a x1 or x2 slot, but with reduced performance.
AGP x4 cards can go into a x8 slot, but without benefits.
AGP x8 cards can go into a x4 slot, with reduced performance, but Cannot go in an x1 or x2 slot. *1

PCI-E 1x cards can go into a x8 or x16 slot, but without benefits.
PCI-E 8x cards can go into a x8 or x16 slot, but Cannot go in a x1 slot. *2
PCI-E 16x cards can only go into a x16 slot, but Cannot go in a x1 or x8 slot. *2 *3

*1 - Except by using a Modded BIOS, special adapter or very costly tools.
*2 - Except by using a cutter or an PCI-E x16 to x1 low-profile adapter.
*3 - Sometimes x16 slots are throttled to x8, but they are physical x16 slots, and should work.