Are you sure you have the versions correct?
Leopard is 10.5
Snow Leopard is 10.6.
All new Macs have come with Snow Leopard for a couple of months now, so unless you have bought pretty old stock you should be ok. Also Apple were doing free upgrades for computers bought within a month of Snow Leopard coming out ( I know because my upgrade was free)
For each release of the OS you'll see there are periodic updates, so Snow Leopard 10.6 will be incremented 10.6.1, etc through Software Update automatically.
At some point the next release 10.7 will be designated and released - 10.7 is already showing up in the web analytics of some sites so its on the way but probably won't be formally announced until WWDC in July and released late '10 or even '11.