Whether they will work together depends on several factors, but, primarily what motherboard you have.
Some motherboards do not allow you to mix RAM sizes/types/speeds. The easiest way to find out is to try it and see.
You may need to try different combinations of which RAM module you put in which slot.
I'll assume you have your 256's in slots 1 and 3, thus:
[256][---][256][---]
First download
memtest86 and
test you current RAM (to make sure that any issues aren't being caused by it).
Now, install the 1GB DIMM into the first spare slot (slot 2), thus:
[256][1GB][256][---]
Test with memtest again.
If you get errors, try the other free slot (slot 4), thus:
[256][---][256][1GB]
Test with memtest again.
If you still get errors try moving the 256's over to the 2nd bank and put the 1GB in the first slot, thus:
[1GB][256][---][256]
Test with memtest again.
If that still doesn't work, try the 1GB in slot 3, thus:
[---][256][1GB][256]
Test with memtest again.
Then, if you still have errors, remove the 256s completely and put the 1GB back into slot 1, thus:
[1GB][---][---][---]
Test with memtest again.
If it still doesn't work, the 1GB module is faulty, but hopefully you'll not get this far before finding a stable set-up.