"Sapphire R9250" or "PNY Verto GeForce FX 5500"

blinky

All modern videocards have hardware MPEG2 decoding built in (sometimes need to enable an option in your DVD software.

The problem with onboard video is that the the graphics chips wants to have access most of the time to the memory preventing the processor and other devices accessing the memory. Additionally, most of the on board graphics solutions aren't very powerful so many graphics operations are offloaded onto the processor.

Putting a graphics card in will probably help more than more memory.

Adding extra memory would probably be a good idea but not knowing what type of memory is required I cannot say if it's a cost effective upgrade.