Crucial.com, as well as the Compaq website will tell you what is appropriate..But if you already know that it came with PC133, then you'll probably need to stick with that. Likewise, assuming 512 MB is the max it will support, (that's very likely), and depending on how many slots are available, you'll need to coordinate size with slots.
An guess is you only have two memory slots and therefore you'll need to purchase two new RAM sticks of 256 MB each. Or, you can buy a single 256 MB stick, add that to your already 128 MB and have 374 MB...assuming that the 128 MB is installed on only one stick vs two 64 MB.
Still, the model number for the computer would sure help to verify...
Hope this helps and let us know more.
I have a compaq presario 1300mhz running windows xp witn only 128mb of ram. xp is using most of the ram and i cant run some games because of the the low amount of ram. pc133 is what the stick of ram is that i have now, i need to know what is compatible with it or if i have to use the same. also i am thinking of going with 512 mb, can i do that?