In general, the more memory you provide to graphics, the better it is going to perform. The improvements in performance are fairly small, however. I typically use 256MB myself...
Impacts? Well, the memory dedicated is memory that is not available for the O/S to use for other things. The aperture size is a reserve against the memory space and not against the physical RAM available, so no penalty (just make sure that, for best performance, it matches the size of the physical memory dedicated).
Ok thank you for the explanation.
So not too much impact in the case of a single use.
Personally I leave the default options.