I suggest you read some of the posts in this forum, e.g. the stickies and THIS thread. fish meat posted the same question yesterday, and he got some good replies.
If it was me, I'd get an Eheim 2260. It outperforms the FX5 in biofiltration. A bit more expensive, but you get what you pay for.
Personally I don't think the FX5 is a very good bio filter at all. It is however a good mech filter. I'll second the Eheim line, and I personally will recommend a 2078, which is an amazing filter.