Ok, I'll help explain stuff to ya.
Now, the cpu is an AMD Semptron 3100+. The actual preformance is measure by a couple factors, mainly the frequency it works at. In this case it works at 1.8GHz. Now, sure, it looks low compared to intel's, but intels aren't very efficient with their preformance thus they need to have beefed out working frequencies. The cpu you're getting is equivalent to an Intel P4 3.1GHz cpu. So that's plenty fast.
Now, you diddn't actually mention a motherboard. This is important as every computer part plugs into it. You need to pick one that socket 754 (S754). Odds are it'll be a AGP graphics card you'll need to get, so if you do get a geforce 6600, it has to be the agp version, and not pci-express.
Now, the ram says lifetime warranty, but I'm not too sure how good of a brand ultra is... and it's been my experience that tigerdirect sucks for rebates, usally a catch like you have to buy 2 HP products or someother retarded crap like that.. so be careful.
Anywho, thiscomputer seems to be good enough to play D3 well. Btw, check out the last man standing mod, and Opencoop! I highly reccomend the 2!