If you were unsure about components, there are forums out there where people would have literally pieced together the best computer for your budget. There are tutorials out there that can tell you how to build a computer.
The other guy is right. The Power Supply Unit will need to be replaced if you want to put in a powerful video card. Dell and other companies generally give you powerful enough Power Supplies to upgrade things like your memory and adding a new hard drive, but nothing major like a video card upgrade.
So if you know how to install a video card and memory, that's 90% of the battle. The power supply connections are pretty easy, everything can only go into one place, and a good tutorial can really help you. The CPU comes with installation instructions, and it's ridiculously easy to install. The biggest "challenge" is the heat sink, which depending on their design, it can be pretty difficult to get on. Not from a technical stand point, but from a muscle stand point. It almost feels like you're going to break your motherboard with some of the designs.
But it's all really easy.
Since you got this Dell, if you need to upgrade the PSU (power supply unit), you really didn't dodge that bullet. Not only that, Dell uses non-standard PSUs. Fortunately, a company called PC Power and Cooling (I think Corsair now owns them) makes Dell PSUs. So do keep that in mind if you plan to upgrade to like a ATi 4850 or 4870 or higher. Also, if you plan to just rely on the wattage to know if your power supply can handle the video card, that's pretty much ALWAYS a bad idea.
Power Supplies have 3.3V, 5V and 12V power rails. Most of your main components draws power from the 12V power rails, such as your CPU and video card. So, many places list the required amperage on the 12V rail (should be listed) for that video card. Not all brands do, so if you go to a place like Newegg, you can generally find a video card made by a brand (like ATi brand, Sapphire, I think ASUS now makes ATi video card, and a few others) and pretty much one of them will list the required amperage on the 12V rail (you don't have to buy that video card, if the required amperage for that video card is true for that video card, it's true for all in the same brand, for the most part).
And to the people who keep asking why he didn't just get a PS3 or a XBox 360. It's personal preference. Also, not every PC game is out for the PS3 or 360, like Crysis and a lot of other games (like Counter Strike Source).