I've lost count of the number of times I've said this now, but Oxbridge especially look at many more things besides GCSE grades; personal statement, reference, submitted written work, written tests and interviews. As long as the other parts of your application are strong, having one weak area isn't the be all and end all, and besides, 6 A*s and 3 As is brilliant! That would give you a points score of 7.8 at Cambridge. I'm applying there with 1 A*, 5.5 As and 3 Bs, which is a points score of 4.3, so as long as you do reasonably well at AS and are predicted at least AAB at A-level, you should give it a go. All you have to lose is the application fee and 1 choice out of 6 on your UCAS form, but if you don't apply you might always regret it.
As for which schools are 'good' and 'bad', the national average is I think 54%, but it may have gone up this year, so if your school's pass rate is lower than that, you'll probably get some extra points.
i duno bt my bruv gt 5a*, 4a's and 4a's at alevels n jst gt admitted to cambridge for law...so it ink ur results r more than ok. depens on wut u wana study cos if u do law den ur Lnat has to b reali gd his was like 21/24