Ddr2 mobo to ddr3 gpu

As said, system memory and video memory is irrelevant and can run a video card with DDR5 if you wanted so it has nothing to do with your system memory being DDR2.

Uefi ready graphics cards began natively with 7xx series with some 6xx series vendors offering some models an bios update to work with uefi motherboards. The problem with older non uefi graphics cards have if the motherboard and operating system is uefi based, as in Windows installed with motherboard uefi bios is the system wont boot or display anything.

Legacy bios from lga 775 to 1366 work perfectly fine with uefi graphics cards right up to 10 series.
 
I will playsafe in case the ddr5 fails to run on 775 socket. I want to buy 730 may be because it is cheaper and it is less risky. I heard that sustainability is a problem wherein you match the ddr5 to ddr2 mobo
 
I have to say that you have heard wrong. GDDRN has nothing to do with DDRN, one is solely for the use of the GPU, the other is for the CPU. There is no interaction at a specification level. This question comes up time and time again, you can use any memory on the GPU with any memory on the CPU.

The 730 is barely better than modern iGPU's, try to get to a 750/750ti if possible.
 
i have a ddr2/775 system running an r9-270 which is ddr5. never had even a hint of a problem with it.

not sure what you read and where but there really is no isue with the onboard gpu ram at all.

only concern is the possible uefi bios issue which the others have already covered.