In the future, you must include in your posts all relevant information so a potential respondent does not have to make a ton of assumptions in order to address your request.
Your computer Mobo will come with "x" slots for RAM, with a specification of the maximum amount of RAM per slot. Based on your statement, we can assume that you purchased a 1Gb "module" and replaced the 256Mb "module" and, further assume, that your computer is recognizing the full 1Gb of RAM that you installed. If this is correct, then we can further assume that you have more than one slot, so you can certainly plug-in the original 256Mb RAM into the second slot.
In general, adding more RAM is one of the lowest cost methods to increase your computer's performance. This is based, of course, on the assumption that the utilization (i.e. running programs, services, and processes, in foreground and background) of your computer is taxing the amount of the installed RAM. Since you have not indicated that your computer is slowing down, all that can be said is that the addition of the 256Mb of RAM won't hurt, but it may not show any increase in performance. Just install it and make sure your computer recognizes the full 1.256Gb of RAM.
The following link will provide a download of a freeware program called "Memory and CPU Observer" which will monitor your CPU and RAM usage so you can see their ongoing utilization:
http://www.freewarefind.com/archives/memory_tools.html
