|"Is this a good graphics card to play StarCraft II with?"|
The 9500GT is not a good gaming card. The system requirements for StarCraft II Beta aren't all that high so it would definitely be playable, but if you have to buy a new card anyway, I suggest you get something better.
The type of memory used on the video card has nothing to do with the system memory. Generally speaking, when looking at video card memory, GDDR2 is better than DDR2, DDR3 is better than GDDR2, GDDR3 is better than DDR3, DDR5 better than GDDR3, GDDR5 is better than DDR5. (G = graphics)
See the following: