Excess memory will slow your machine, the memory controller has more memory blocks to account for, meaning that data stored on the RAM takes longer to retrieve. In addition, the RAM will get fragments all over the place, not to mentioned added latency.
Memory size needs the proper speed to compensate, same deal with VRAM, no point to buying a 6200 with 512MB if it can't utilize the memory properly.
BTW, I'd take an LCD projector over the grand canyon, possibly one of those three colour projectors (can't remember what they're called).
