Change my Shimano SIS rear derailleur of 7 speeds to a better one?
You could use any Shimano derailer. I would go for Acera or Alivio, since they have good quality and good price.

Some SRAM might work, but beware that SRAM and Shimano have a different "cable pitch" (amount of cable pulled for a given amount of lateral movement of the derailer pulley). Even though, SRAM manufactures a few models which are compatible with Shimano levers, and also some shifters which are compatible with Shimano derailers.

Also, if the "cable pitch" of the system is correct, you only need to match the cassette model and the shifter model (in your case, just keep your cassette and your shifter).

Hope this helps

EDIT: if it is a road bike, perhaps Acera and Alivio couldn't be the best choices, but I'm not up to date with Shimano options for road derailers. Anyway, they are interchangeable regarding "cable pitch".