A few points:
i) If it has become slower over time then maybe, but there are probably other things you can do first. e.g. run a defrag.
ii) I would always recommend 512MB as a minimum for all but the most minimal users of Windows XP. Yes you can get away with 256 but you will find that Windows 'swaps' (uses the hard disk as memory, in essence) a lot. Do you know how to get to task manager? If so, look at the performance tab for a figure called 'Peak'. That will tell you how much memory your machine has used at peak. Memory is quite cheap nowadays so it is probably worth doing.