Your Motherboard has 1 PCie x16 slot so you can plug in a GTX 550Ti. However, the PCIe slot in the mobo is very likely NOT a PCie 2.0 compatible slot. Therefore, you may not be able to get the GTX 550Ti to work. Sometimes there is a BIOS update available to make the slot work with PCIe 2.0 devices but it will still operate at PCIe 1.0 Speeds. I checked the HP website but did not see any BIOS updates that addressed a PCIe compatibility issue for your mobo.
Yes! I pretty much gave up on it. I've even upgraded the power unit. The first thing that I did was look up for a bios update. When I found there to be none, things started to look a little dim. Then when I installed it, I notice right away that the unit was not being recognized. So I figured that the slot just wasn't PCIe 2.0 ready. So I knew that That was pretty much it. Thank you much for the reply, and conclusions matching confirms the outcome. It is time for an upgrade. Maybe I'll just gut it out and rebuild form a new motherboard up. Again, Thank you!