Personally, I'd like to say to buy the HP, as they put dinner on my table every night
I built my own image editing desktop out of a Dell workstation. Dual quad core xeons, 16 GB of RAM, multiple 10k RPM hard drives etc, but that was mainly because I'm geeky and I like geeky stuff.
Either brand will be fine for what you're doing. Definitely try for 8 GB of RAM (or more) - I run 16 GB in my ASUS laptop and my Dell workstation, the laptop came with 6 GB and I found with Lightroom and CS5 both running, it got real slow.
Regardless of which you choose, considering what you're starting with, it will seem like you're going from a Chevy Cavalier to a Corvette.
If you feel like it, post the specs of what you're getting with the price you'll be paying, and perhaps someone here can find you a better deal on something similar or faster. Sam's often has good deals, but they usually aren't the *best* deals!