Hmm, well it depends on if you're using clsoe to 512 MB of ram. If you are then keep it, if you're only using ~300, which you probably are using at boot, then kick out the 128...But I mean, it's 100 vs 133...time to upgrade my friend
Your on the ragged edge of no go. The amount of speed difference between pc100 and 133 is minuscule but adding a 256 or 512/133 in place of the 128/100 should give you a somewhat noticeable increase. Don't spend good money doing this cause you should be saving up for a four generation newer box.
Either way will be a bust. The difference between 133MHz and 100MHz, assuming the motherboard even supports 133MHz memory bus, is going to be negligible. The difference between 640MB and 1GB, also negligible. You don't have any shared graphics memory, and have over 200MB RAM available for use even with IE 7.0 and Outlook Express running. You're not running "low" on available RAM, particularly for Windows XP. Anything more 512MB on Windows XP begins to court the law of diminishing returns, unless you are running applications that really need it (and it doesn't appear you are).