To echo what MottiKhan said, think of a computer's architecture like a pyramid. The better the base, the more stable the outputs at the top.
For a while, new RAM and video cards and whatnot can be used to gain an improvement, but after six years? Well, those parts are going to be putting a stress on your basic components, specifically the motherboard and power supply.
For example, one of the best graphics cards out today, the Nvidia 9800 GX2 with SLI, has a list of supported power supplies. Slight the power requirements and you could get less performance, or worse, a fire. At the same time, the GX2 runs extremely hot, and if you don't have proper cooling systems it could lead to system instability. AND, if your base motherboard architecture is old, you're not going to get the most out of that card.
That's the problem with boxed systems like a Dell: the parts that are put in there are sufficient for those parts alone. You're not going to be able to do much more with the motherboard that's in there because it was designed for that system, and the power supply was chosen to meet the requirements of what the system was built with.
As another example, I just helped my parents upgrade the RAM in their Dell. They tried to do it themselves and were getting system errors, and I found out that particular model could only use TWO types of RAM. They had bought something that was too fast. Of course, the available RAM was at the same speed as the old ones, beause higher speed meant more power that the motherboard and power supply couldn't handle... all they could do was increase the capacity, moving from 768MB to 2GB.
So.... six years? I'd recommend a new PC. But do yourself a favor, in line with what others have mentioned: Spec out what you want, and then wait 3-6 months. You could save yourself hundreds of dollars.