The best I can suggest is not buying a Dell to begin with. Build your own or have someone you trust build it for you. You will get better components at the same or less price of a Dell.
best advice... +1 
Highly agreed +1
Here's a "why?": Dell and any other widespread brand don't make dedicated gaming computers. Their idea is to standardize to the max to decrease manufacturing costs. In the lower price categories that means using outdated and/or underpowered components, balancing the rig so that nothing can be upgraded without having half of the innards changed. Their "professional" models are of better quality, but they are stripped to the minimum, often even lacking a graphics card slot.
If you don't want to build yourself, look for either a local builder shop or an Internet builder, who give you the option to make changes to their suggestions. I've seen sites that have literally dozens of slightly differing options, starting from a budget rig and ending to a gaming monster in increasing order. Such sites make it easy to figure the total cost and the cost effect of replacing only one part. Unfortunately I can't give you any addresses, I only know one reliable net shop here in Finland and one good looking in Italy, but do some research and you'll surely find one at your whereabouts.
