If the HP is a fairly recent model, like within past 4 years, the mobo will be standard. If it is older, probably not. You will need a mATX board (micro ATX).
The DX10 cards are all PCI-E, so you'll have to have a board with a PCI-E slot.
You'll need to relpace the PSU, though, even if you re-use the case. Most pre-builts come with a 250W PSU, possibly a 300W... not enough to run newer cards.