You'll need the same type of SO-DIMM as you have now, but in larger capacity; Notebooks all come with 2 RAM slots, both filled, so that when you upgrade your RAM, you have to throw out the old stuff You can use a single stick of RAM if you want, dual-channel does not, to my knowledge, exist for notebooks.

Per HP, you're rocking DDR2 @ 667 MHz, so it should be pretty easy to find more of the same. The clock speed can be higher if you want--the computer doesn't support more than 2 GB, so the rule about the RAM operating at the speed of the slowest SO-DIMM goes out the window; If you put a 2-gigger in there, you can't put the 512 in as well. If you do, the system may only recognize the 512-megger. Hence, if it's cheaper to get 2x1GB than 1x2GB, go that route, at the fastest clock speed your budget allows.

It's actually quite ironic--about the same time you posted this thread, I was trying to get rid of 2 1GB sticks.