My advice would be to use it for the new system OS, Apps, some progs.
I have the 64g Crucial, and only about 20g is used for Win7 64. Greatly improve boot time and load time for newer games. Don't really see a need for you to spend money to speed up your old system boot/loads if you won't be using it much. Besides that, new systems always tend to make one apt to buy new stuff for them.