I've installed several different power supplies in HP cases (same case as yours, but different models) and yes it will fit and yes it will work with no modifications. It will sit fan side down in your case. I recommend a higher quality power supply though for only a bit more after rebate. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
PS. If you're adding a long graphics card into the case you may have to move a couple of SATA cables to different sockets, but it's easy to do.
Do you think 550W is enough for me though? Along with everything that's listed on the site I linked, I will have an AMD 6400+ 3.2 GHz, NVIDIA 8800 GT OC, and 4GB RAM.
edit: SATA cables are the L shaped ones, right? I don't think they're close to my video card (I already have the 8800, I know I shouldn't with 250W, but it's been fine for about 6 months, so I think it's okay).
I have the a6110n and MANY people have told me that the wattage upgrade has no limit. I want to be problem-free with The Sims 3 game so today, I bought the NVIDIA GeForce 9500GT with 1gb of DDR2 with PCIe x 2.0 which requires a power supply of 350watts. The a6110n only has 250watts so I also bought the StealthXstream 600 Watt ATX 12V Power Supply to replace the old one. I am really hoping I dont fry anything by doing so but MANY people have told me it should be just fine since the computer was made in 2007. I am ecstatic to get home and install these items on top of the 2 more gb of RAM and am just REALLY hoping that everything flies! Please let me know if this will work or not...?