Yes, it would be worth it.
SATA II (3Gb/s) SSDs are "old tech". You should get a current generation SATA III (6Gb/s) SSD even though you will be connecting it to a SATA II port. Current generation SSDs will last longer and are more reliable/compatible.
Since its a old machine, should I only get the 64bg one and only put the OS and maybe one major program on it or should I go with the 120 gb one?