Well the X1550 has the better graphics processor, so I would go for that one definitely. The only plus of the other card is that it has a large onboard graphical memory, but I don't think it will ever be used. I explain:
Onboard video memory is mainly used a buffer. When the card is processing a frame, it loads it up in its memory and works on it there, since working with VRAM is much, much faster than using system RAM. For higher resolutions and/or high quality effects, you need more memory.
However for a card such as this one, I wouldn't expect running games at very high resolutions and/or very high quality since the GPU just can't handle that much, so having 512MB of VRAM is useless.