I googled "Compaq Presario 4400US motherboard" and arrived at:
http://www.memorystock.com/memory/CompaqPresario4400US.html
... which says (in translated Chinkery, it seems):
"Compaq Presario 4400US takes the PC133 SDRAM DIMMs Memory Type, and comes installed with 128 MB (removable) of Memory. You can upgrade your Compaq Presario 4400US to up to a maximum of 512 MB Memory, the system has 2 sockets to install Memory, already with 128 MB (removable) standard Memory installed."
This tells me you have 2 memory slots, which at this moment is filled with either 1 empty and 1 module of 128MB PC133 SDRAM DIMM, OR they are filled with 2 of the 64MB modules. You still need to open the case and see which is true, but largely you're in good shape for upgrading.
If you have the 2 of the 64MB modules, you'll need to pull at least one of those, since you'd be replacing one with a 128MB module at the minimum. This is not as effective for the money, since you want to get to at least 256MB ... but it will be your cheapest option. If you have the 1 of 128MB module only, then it'll still work and will get you 256 total.
A 256MB module should be fairly cheap. The link provided offers one for $32.95. So, if you can spare a memory slot, adding one of those would about triple your memory (128+256=384, or 64+256=320). You should be able to mix them. Out of convention, place the larger module in the slot marked 0 or 1 (such as "DIMM0"), on the off chance that the memory-access circuitry is slightly biased towards the 1st one.
When buying, it's best to bring in a sample module, but you already know you need to buy a "PC133 SDRAM DIMM". Any computer store clerk will know what that is.
Needless to say, be careful handling the modules since static can harm them, and of course don't physically mess around with memory modules with power on. (You can leave the unit plugged into the wall power, as long as the computer is OFF.)