Cardiff secondary schools

[pokemeinthemorning]

I currently live in Newport and have been here around 3 years. We have a nice house but I'm looking at moving for the children's education. Most of the schools here are pretty dire (in particular secondary) with low rates of achievement at GCSE. My eldest started secondary last year and I have another 2 who are currently in year 5 and 3.
My middle one is quite academic and I've been told she is MAT by her teachers but the school she is in (along with her brother) is not great either with a high turnover of staff which is quite disruptive.
I really want to try and give them the best chance and am thinking of relocating.
I just wanted to know how easy the better Cardiff secondaries would be to get into if we were to move possibly next year.
I'm not after the best school as we'll not be able to afford the house prices in those areas but ones which are performing better than our current ones where housing is affordable.