Checkpoints - what happens actually?

Hi,

i went across a checkpoint problematic (90% load, near real time app). I've
read in pg-documentation what exists about CHECKPOINT and the configuration
parameters. But there're still many open questions as:

What actually happens if a checkpoint occours?
OK, all dirty pages of data files get written to disk...but

Are read requests (simple selects) still possible and quickly answered?

What happens to wite-operations? Do they get queued or can they be written
to xlog while the fsync is running?
Is it maybe really the case but wave of pending statements arises from the
reduced i/o capacity which is left due to the fsync?

How about increasing the interval more and more? Will this produce more and
more dirty pages or is it just that a possible restart will take a longer
time since the xlog is longer?

Questions over questions. Can you please give me a clearing little insight.