How much of a performance hit using asymmetrical dual channel RAM?
The motherboard will fall back to single channel mode. See this TechSpot forum post: Dual Channel with 3 sticks of RAM... This is why a 6GB configuration is typically 2x2GB, 2x1GB.
However, as per the Wikipedia article on dual-channel architecture, depending on your applications, you may or may not experience a significant performance difference. Memory-heavy operations may be noticeably slower, but don't expect a very large difference.