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).