Ciao - Gio Love Italy!
Yes, you can swap the HD out to a SSD with little effort. The tricky part is installing the OS. Do you still have the OS-9 CD/DVD's that came with the system or have the bought OS-X DVD's? Any will do. If not you'll need to find a way to link your system to an external drive (see below) or another Mac to prep the SSD (target mode).
You'll need an ESD mat & wrist strap plus the needed screw drivers (Straight, Philips & Torx).
You may want to consider getting a FireWire (getting hard to find) or USB HD case to put your old HD in. Or a USB to SATA cable so you can copy over the apps you have on your HD.
One other option is taking out the optical drive and with a optical to HD carrier swapping out the optical drive for your HD. That way you have the SSD as the boot drive and the HD as a data drive (no internal optical drive anymore). If you want you could still put the optical drive into a FW/USB case so you have it when you need it.
Follow these guides:
Dual HD install