There might be a conflict, as it is a laptop, I am sure you need matched pairs of RAM.
Not true. I have a 1GB SODIMM + 512MB SODIMM on my Compaq. The only possible issue is that not having 2 SODIMMs of the same size *may* mean the laptop will run the memory in single-channel mode rather than dual-channel mode. Check your laptop manual and/or HP tech support to find out if your laptop can run memory in dual-channel mode, and if so, do you need to have same-sized memory modules to do so.