Dual booting Snow Leopard with Leopard

[stealthly]

i am looking to Dual boot my Intel iMac with Leopard (10.5.8) + Snow Leopard (10.6) for testing purposes.

 

How do i go about doing this?

 

I have the Leopard install disc

 

i do not have Snow leopard yet and the only version i have seen is an update disc you can buy from apple for about £25 is this ok to use or will i need a copy of the full version?

if i need a copy of the full version where can i find it?

 

my current hdd setup is:

1 partician very large for data about 255GB

+ another 40GB Partician with Mac OS X

 

I assume i will need to create another partician about 20GB and call it Snow leopard and use the leopard disc to install onto the new partician then upgrade the newly installed leopard partician with the new snow leopard disc??

 

Thanks in advance

 

Regards Sam