Can I install 10.6 on a macbook 4,1 using a USB optical drive The superdrive in my late 2008 4,1 macbook has passed on but I need to upgrade the HDD with a larger one meaning a reinstall of 10.6 snow leopard.

Can I use an external optical drive to do this on such an old macbook or will I need to replace the internal?

If I can what key do I need to hold down at boot to access the USB optical drive.

Alternatively is there some way I can make a bootable USB, preferably without having to upgrade to Lion.

Thanks.
Yes, you can.

Holding Option on boot, will show you all the bootable possibilities.

Making a bootable USB is as easy as making a clean installation. Format the USB with the correct Mac Os X format, and then install as usual. Here is a step-by-step guide, with lots of useful tips.