Can you tell us what version of OSX is on your machine and what Leopard did you are wanting to use to load your machine?
Can you click the apple icon on your machine then click about this Mac, as if your machine meets or exceeds the quallifications for Snow Leopard, Then yes you could legally get the disk for $30.00 and upgrade all the way as that is what Apple intended the disk to be used for.
Let me know so I can check your specifications
That is more than enough to run Snow Leopard as the requirements are a Mac computer with an Intel processor and 1GB of memory with 5GB of available disk space and a DVD drive for installation so you are good to go.
So all you need to do is pick up the $30.00 Snow Leopard upgrade disk from Apple or from your local electronics retailer http://store.apple.com/us/product/MAC_OS_X_SNGL
Take a look and let me know if you need more info or anything else