Ditch the code when you post sites, it makes links for you.
Hmm, if I could see the power supply I could tell you what it was. When I had a a810n from HP it was just a standard ATX power supply. I kind of doubt that they'd randomly use something that old. You really just need to find out what it is.
If you tell me what the CPU you got is I could recommend something but if it's a dual core sufficiently fast, then your choice of a 5770 would be fine.
Hmm, I used the name of it on the HP site and just searched youtube for it. It isn't ATX at all I think it may even be a BTX. Good luck with that. If I were you I'd toss off an email to HP support asking what standard it is to make sure before buying something. Though I'm betting changing the PSU voids the warranty also.
What's the current power supply capable of?