Do I need to disable integrated graphics on my hp?

you usually don't have to

you should be fine just go ahead and add it in

Is the onboard also an ATI chipset ?? - IF not it would be best to go into windows and remove the current driver and switch to the generic VGA driver. (not really needed but makes sure you do not have to boot into safe mode to add the new card when it doesn't find the onboard any longer) - also you can go into the BIOS and disable the Onboard (again usually not needed since when you connect the monitor to the new addin card it will normally change it automatically but doesn't hurt) and change the default to the PCI-e slot.( so it doesn't waste time during boot trying to initialize the onboard or PCI bus first) - save the changes and shut down then put the card in.

These steps are usually unneeded ( over the years systems have become more plug and play ) but it does provide some insurance that things will work properly when booting with the new card - so it doesn't hurt !