512MB should be considered the minimum for XP. It will run much better with 1GB. I would hold off on the HDD upgrade. Do you have a video card or are you using onboard video? Upgrading to a card, even a cheap low end one, would be better than using the onboard. That way the system can use ALL the RAM.
A larger hard drive could make your computer faster if you increase the size of the swap file and if you get one with faster read/write times. That would probably be the case with a newer hard drive-and your current hard drive might only be 5400 rpms-the newer one would likely be 7200 rpms(though some newer large"green" hard drives are 5400 rpms) I agree with the above responses about increasing the RAM to 512 MB-but I don't think that 1 GB is that critical for XP. Of course, it depends on what and how many programs you are running, etc.
I would get a stick of 512, unless there is some dual channel issue I am unaware of, the price difference is not that great. I just installed xp yesterday on a dell with a celeron & 512. By the time you install an av & firewall, you are using a large part of the ram, as soon as you power-up.
In addition to the comment from larryf215, modern browsers gobble up memory too. The difference between 512Mb and 768Mb is astounding and often the difference between a slug and a machine that works well.
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