I did exactly the same as you, with a 320GB drive too.
As well as buying a new HDD, i bought a HDD USB caddy, so i just put my new hdd in the caddy, and told Carbon Copy cloner to clone the Mac HD to the Caddy HD.
Took HD out of caddy, swapped over the HDD's in my MacBook, all fine! It did for a split second show the folder with a question mark in it, but that literally went in a split second when it found leopard.
The only warning i would give with this is that it messes up your Boot Camp partition so if your running windows you may have problems there and may need to re-install.