how to reduce rsync system usage

[dtra]

hi all

we're using rsync to backup our web server at the moment using it in a 
cron job
the server we are using to receive all the sync files is a live but not 
heavily used linux machine
but rsync still takes up more resources than we want it to
it takes up to 95% (fluctuating) cpu load and a fair bit of memory too

the cron job uses nice -19 rsync
but that doesn't seem to do anything, is there anyway to make it use 
like 5% cpu or something?

thanks
dave