good program to model 3D changes to protein from mutation - (Jul/18/2006 )

i've been looking into findiing a program - preferably available or downloadable online - that can make two theoretical/predicted 3D models of a protein I'm studying - one with each allele of a non-synonymous SNP (i.e. either an arginine or a cysteine at the same position) so i can compare them visually, and in an ideal world, statistically on some level. Any suggestions or personal favourites?

Thanks!