|You currently have a Dell, which uses a version of Vista that is keyed to the motherboard. So, upgrading the motherboard and CPU would most likely render your Vista unusable.|
Better to leave that computer intact and either buy or build a completely new machine. You can reuse monitor, keyboard and mouse if you wish.
I am not a gamer so I can't help too much with selecting a new rig.
Building a custom computer is usually more costly. Primarily due to the cost of the operating system (Windows). OEM manufacturers pay only a fraction of what us end users have to pay. However, full versions of Windows have more flexibility to migrate to new hardware. The full retail versions have the most. You can migrate them to any computer that will run them. The rule is one computer per license.
Many times people will purchase an OEM pre-built computer based on the price. Then turn around and upgrade the graphics to a high end gaming card. This in turn requires them to replace the power supply, which was sized for the marginal hardware that came from the factory.
Another pitfall are these computers with smaller or slimmer cases. These require different hardware when upgrading and sometimes upgrading isn't even an option.
Hope I have provided some incite on the pros and cons of what to buy.
You need to set a realistic budget for whatever you intend to do and then others can help with selections.
If you decide to stay with an OEM then order the unit the way you want it to be in the end. That way, if a more powerful power supply is required it will be in there to start.
Most of the OEMs have direct buying sites where you can customize one of their base models to get exactly what you want.