Corsair, PC Power & Cooling, and Seasonic are some of the brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty. Some of the newer models come with a 7 year warranty. Lately we've been seeing a few other brands offering some high quality units. An example would be the Antec Earthwatts series which is an improvement over Antec’s older series psu’s.
Thank you for the advice, I have little doubt that the supplies you guys suggested are all better than the Ultra, I had already ordered before each of you had responded, at least the Ultra has a Lifetime replacement warranty if I register it. I am kind of stoked though because the stock supply was only 300 watts and now I can add a really decent graphics card, the PCIe slot is a 1x16, not the newer 2, see any real limitations there?
The Ultra you selected should work but I would recommend a Corsair, Antec (except BP's), Seasonice or OCZ PSU instead for a more reliable PSU.
All of the answers were helpful, but knowing that a standard ATX supply would fit in this HP case helped the most. If I were building a PC I would definitely look at the makes that you guys suggested. I would also be looking at a different build altogether.