Andrewas is right. Unless you have Windows 2000 or XP there ain't much point in having more than 256MB of RAM. Also before andrewas posted I was gonna say that it's unlikely that your mobo would take DDR RAM. In case you are wondering, DDR is Double Data Rate and thus is accessed twice as fast as SDRAM (I think it's Double compared to SDRAM) that's why it's so much more expensive. Unless you are doing some serious video editing or running 2 million applications at once, it's better to go for the faster as opposed to the more RAM, but you don't really have a choice. ALso check that your system will actually run PC133 and does not require PC100 as MY 2 year-old system failed to boot when I bought PC133 RAM. THEN I looked at the manual and it told me that I need PC100 so I went and exchanged it
Teach me to read the manual
Anyway, windows 2000 runs my 640MB RAM nicely. The PC came with 128MB then I added 2 256MB DIMMs. So that answers your other question: make sure you do not have to throw away your old mem module, that'd be pointless.