The speed of the memory card is definitely one constraining factor but as you suspect there are other bottlenecks. First there is the internal memory buffer of the camera. Each camera only has so much RAM installed. When you shoot this buffer is filled first and the camera does what it can to quickly empty the buffer to allow for more shooting. The size of the buffer generally correlates to the price of the camera. A professional DSLR will have a larger buffer than a consumer DSLR.

The size of the file being recorded affects how many images can be in the buffer before you experience the slowdown. So capturing 14 bit raw means you hit the wall much faster than 8 bit size optimized jpeg.

So that leads to your main question: will a faster memory card help in burst mode. Yes, it helps. But only to a point. Keep in mind that writing to memory in the camera is orders of magnitude faster than writing to a flash memory card. So having a faster card gets you a little more burst but not as much as you'd get from capturing significantly smaller files.

I'm mobile right now and can't do the math but my experience with UDMA and slower cards suggest that the faster card helps but it isn't a night and day experience.