f stop associated with AF point

I will be grateful to be enlightened about the concept of associating an f stop with an AF point in the rebel and other canons!

The sensors in the 350D are designated f stop 5.6! Does that mean that low light situations needing faster Fstops will make the cam search for focus?

The 40D has cross type AF points and have f stop 2.8 associated with them [ in addition to other AF points having the value f/5.6] does that mean low light shots needing wider aperture [ like 85mm f1.8] will leave the cam searching for focus?

I unsuccesfully tried searching this topic , hence the query!

thanks in advance for the inputs that may come