Are you sure you have a Dell c521? All information I can find on that model indicates it only supports AMD procesors. In any case, the problem you have is the Pentium 4 CPU no longer meets minimum system requirements for pretty much any game released these days. Battlefield 3 and Modern Warfare 3 require a dual core at minimum. A single core with hyperthreading, is not going to be enough to run either game. I'm not sure if your motherboard will even support a dual core CPU. You'll have to check the documentation for a CPU support list, and see what your upgrade options are.
You will also need a new Power Supply, which might be a problem, as Dell sometimes uses proprietary power connectors and PSU form factors, meaning you can't use a non Dell Power Supply on your machine. 280 Watts isn't enough for any decent video card. A 5670 requires at least a 300 Watt Power Supply. The best card you could run on your current PSU would be a Radeon HD 5570, which probably will struggle with Battlefield 3, even on low.
Honestly, with a 250 dollar budget, you might as well go out and buy a console to play these games on. You're probably going to have to build a new system in order to play these games on PC. You probably won't be able to upgrade that Dell to be able to play MW3 or BF3, or even if you could, it would be quite expensive, and not really worth the money just to get a machine that can barely run the games on low settings.