ok first off, you bought a HP so you already got screwed over.
If you decide to buy a new motherboard, you also have to buy a new power supply since HP has a different power supply.
Now, if you bought a radeon 9800, somehow i believe it wont be used to the full potential it has if your using that HP motherboard.
As for your HP motherboard, and since its a 1.7, im guessing off the bat your motherboard FSB is 400, so you probably wont be able to go that much higher if ur upgrading the processor.
Id say, buy new mobo/processor/PSU.
Oh yea, as for the FX chip, heres my 2 cents.
-Highly Useless Currently inless you plan to be running a server or something.
Even P4's with 800 FSB is useless imho inles your running a server or video editing or somethin