$1500 will get you a very nice desktop. However, your question will be easier to answer if you tell us what you'd like to do with your computer.
For a balanced, do-anything system, I would suggest a true quad-core processor. The Core i5 2500k is probably the best bang-for-your-buck processor in the middle-to-high price range. You'll get most of the performance of an i7, but at much lower cost. You can read more here: http://www.anandtech.com/show/4083/the-sandy-bridge-review-intel-core-i7-2600k-i5-2500k-core-i3-2100-tested/22
If you plan to do any gaming, you'll want to reserve some money for a good video card ($160 - $200). You'll probably get a better deal if you upgrade the video card on your own (rather than through Dell), but that depends on your comfort level with installing parts.
As for the hard drive, I would suggest a small solid state drive for the operating system (at least 60GB, larger if you are a game and want your games to be installed on the solid state drive - in that case you'd want to go for 120GB).
Then get a secondary hard drive for music, movies, photos, documents, etc. Again, I'd install this on my own since you can buy a 1TB hard disk for $60 from Newegg.com. Dell will probably charge you much more.
As for memory, I'd go with 6 to 8 GB. There's no reason to get less than that since RAM is so cheap these days. Again, RAM may not be cheap through Dell, but it is cheap to buy in general. My apologies, but you can tell that I like to build my own systems. There's nothing wrong with off-the-shelf (sure saves a lot of hassle), but sometimes they really stick it to you when you upgrade from the very basic configurations.