Getting A*s in GCSEs won't have much of an impact on your uni application. You just need to do well so that you can do the relevant A-levels, which usually just require a B or a standard A. You'll have to ask your school about their requirements for A-levels.
Once you're into A-levels, getting A*s will matter more, because unis will want to see you do well in your A-levels. (so basically, unis look at your A-levels more than GCSEs :P)