Putting the swap file on a HDD will not stop your computer sleeping. What stops it sleeping are constant access to either memory or any type of drive (SSD or HDD), or network access.
Some programs, like MSN Live, steam, online games etc that are constantly doing stuff often will keep a computer awake.
I have an OCZ SSD. I have 8gb RAM and a 1TB HDD.
My computer is set to go to sleep in 10 mins, monitor goes off in 3. When I moved my swap file to my HDD (and there was no compelling need to with 8GB RAM), it made not the slighest difference. The computer still goes to sleep.
There are even some programs I can leave running, like jdownloader, and the computer will go to sleep.
Some green drives are designed to use less power and will spin down when not in use. These are typically slower rpm drives, like 5400rpm. You will notice a pause when trying access data on drives that have spun down as it takes several seconds for them to spin up again.
You can also make some adjustments in control panel power options, <power plan>, advanced settings.