Buy the SSD* and a cheap USB 2.0/3.0 enclosure for 2.5" S-ATA drives, they cost 10 to 20 € nowadays.
Put the SSD into it and format it properly**.
Then either download
CarbonCopyCloner (version 3.4.7 is still free and available for download and works with
Mac OS X 10.6 Snow Leopard,
Mac OS X 10.7 Lion and
OS X 10.8 Mountain Lion) or
SuperDuper! and clone the HDD to the SSD.
Then restart your Mac and hold down the OPTION/Alt key to select the external SSD to boot from, to see, if everything worked alright.
Then switch the HDD with the SSD and put the HDD in the enclosure for backup or storage purposes via formatting it via
Disk Utility again, if you want to.