See this is the thing about ordering Dell's. If I was building one, I'd go for the best possible MB that I could get my hands on, but for the need to save money, I'd go with the lowest processor that board would support, along with memory, etc...
That way, later I could upgrade processor and memory, and have a fast machine. With Dell, they ship different boards depending on what processor you order, and therefore send you the cheap board with the cheap processor, along with outdated memory support. That is one major drawback about ordering pre-built machines (all of them, HP, Dell, Apple--which you can't build your own so--lord forbid Gateway, or Systemax.
Anyways, if money isn't an issue, I'd really go with at least the higher end Dimensions (don't have to go all out with XPS's or anything). Perhaps not the most expensive, but don't get the cheapest for sure, you'll get what you order. Spend about $1000 for computer (plus monitor) and you'll likely do alright. Worry more about price range than what processor (price range doesn't include extras like 3 year warranty, 21 inch monitor, 15 software titles, etc. just the computer only--with plain old cheap Dell keyboard and optical scroll mouse and cheap speakers you should spend about $1,000, plus all of your extras) the computer has in it. If you do that, you'll be a lot more happy.
However, if price is an issue, don't buy the cheapest computer, go for one that is above the bottom price range, if you don't, you will not like what you get.
Good luck, and if you've already bought one, congrats.