Is it possible to have AMD AM2+ mobo with 790fx chipset and still have DDR3 Ram support? Also do you think DDR3 is worth it? I like this one but it doesnt have DDR3 support :(
Short answer: No.
Long answer: AM2+ processors only support DDR2, so any board with that socket will have DDR2 to maintain compatibility. To have DDR3 support, you would need a AM3 motherboard.
It looks like the 790fx chipset only support DDR2 also. I assume you mean the 790 if you mention a fx chipset?
The only 740 is the 740g chipset: http://www.amd.com/us/products/desktop/chipsets/7-series-integrated/Pages/amd-740g-chipset.aspx
DDR3 is the new standard for RAM. DDR2 prices may fall and will eventualy rise some as it is phased out over the coming years. Most of the small home servers and small notebooks still use it though. If you are upgrading a current system and using an older processor and parts, then DDR2 is your only path. If you are building a new machine, I would go DDR3. It is getting cheaper by the day, faster in many aspects, and most newer boards come with it.
If your trying to build a super cheap system, then DDR2 might be a better option.
What CPU are you going to use? Chances are it is AM2 or AM2+ and only supports DDR2 (memory controller is integrated in the CPU) making this question moot.