OK, on that ASUS board you have onboard video (S3 unichrome) but there is an AGP slot, so that AGP card would fit in just fine. When installed it will automatically disable the onboard video so just install the drivers for the card and you are good to go.
You will also get that amount of RAM the onboard video was using back for use by the system so the machine should run a little faster with it's full 512 RAM. That's an extra bonus.
HP's have notoriously small power supplies but the fact that this is not a "killer card" is also a bonus.
I think you're good to go with that purchase you planned. Let us know how you like it.