RAM on video cards is used to store textures and files that the card works on setting up for putting on the screen. The larger the memory space, the more it can work on at once without going back to the computer RAM or hard drive. The actual speed of the card affects how fast the card does the work on this. A 9500gt is a slow card, so having a large amount of memory won't do it any good since it will be spending a large amount of time processing the data. Much like needed to empty a lake (in this case video info) using buckets. You give 2 people the same size bucket (amount of video RAM), but one walks half as slow as the other (the 9500gt card), it will take twice as long.
You should get the Radeon 5570, it will perform pretty well, will match your system well (maybe even outrun it a bit but you can move it to a new compter later), and should cost about the same as a 9500 which is several times slower.
Hope that explanation helped with sorting out the video card question. Never ever ever judge a card by how much RAM it has.