Need help with my new computer...

[DingDingDao]

So I won an HP Media Center PC from Microsoft at CES this year, and I'm going to set it up for HTPC use since my last computer build was in November, and I don't really need it as my primary computer.

The problem is this: the HP computer is fine as far as components go, except for the case and PSU. I swapped out the crap Geforce card HP put in the box for a 9800Pro, but the 9800Pro has a power requirement (direct molex connection) that the Geforce didn't. After installing the 9800Pro, I realized that the PSU in the box is only rated to 200W (WTF?!), and it's causing my video card to hang when I try to push the GPU (games, DVDs, etc.). That power supply is not a full ATX PSU (smaller...mini-ATX?), so I've decided to do a case swap, since the plastic case HP uses is BS anyways.

So then I ran into another problem. I don't know if the mobo in this computer is ATX or mini-ATX. The board looks full size, bigger than the mini-ATX mobo I installed in a SFF PC I built for a friend. But it only has 4 PCI slots + 1 AGP. Can somebody enlighten me as to WTF is going on here? Am I looking at an ATX board with 4 slots, or a mini-ATX board? Is there some way to determine which one I'm actually dealing with?

This leads me to my last question. Will an ATX case mobo tray accept a mini-ATX mobo? I assume yes, but please tell me if I am mistaken. Sorry for going on and on, I just want to get this corrected before I buy a new case and PSU. Thanks all.

CLIFFS:
1) Need new case & PSU
2) How can I tell if my mobo is ATX or mini-ATX?
3) Will an ATX mobo tray accept a mini-ATX mobo?

 

[mbackof]

What is the model of HP machine you have? I can look up the tech specs and get back to you with an answer on whether it is micro-ATX or ATX.
As for your other questions.
Usually an ATX case's motherboard tray accepts ATX or Micro-ATX motherboards and has the standoffs for both types.
It certainly sounds like you need a better power supply. I believe that HP is using standard ATX power supplies now, but I would need to look that up when I get your model number.

 

[DingDingDao]

I think it's an HP Media Center m390n. I threw out the box, and there's no indication anywhere on the PC (*stupid* of me).

 

[mbackof]

I'm surprised it wasn't on the packing list or bill of sale that came with the PC. Alright, lets assume that is the media center PC. If so, here is the HP Support Page.
The product specs for this are here
It uses an Asus P4SD motherboard which is a microATX form factor. You should be able to replace the power supply in your box and get another one to run this motherboard. Or if you wish, you can switch out the case.