Unless your reasons are unrelated to performance, are you interested in considering an Intel CPU? If not, I'm not trying to provoke a debate, but if you aren't sure, you may wish to check some benchmarks of the [types of] games you play. If you're set on AMD, that's fine; a competent PC can be built with an AMD CPU.
Do you have a monitor? If so, what resolution? Do you have a legit copy of Windows you can use, or do you need to buy one? Mouse, keyboard, speakers? These are all little things, but if you need them, they'll eat into your budget.