I know this is an out dated thread, but it may come in hand for others.
Windows XP NEEDS at least "512" to run properly.. the easiest way to find out is.... open up your tower and take out a ram stick , it will say on it what it is. But before doing this, please turn off the PC and for safety reason UNPLUG the power cord.... If you have all slots full .. and your not running a " celeron" you can upgrade the ram as high as 1 g more than that and XP can recognize it and you've wasted your money.