Image source: NewEgg.
Given the money we have available, you could really go on the cheap and get one of those case/PSU combo bargains - but we would advise against it. The goal is a fast and stable gaming system that will provide hours and hours of reliable game play (this is based on my own unrelenting personal research spent on Battlefield 2 alone). Just remember with those combo cases, you get what you pay for. Many people overlook the importance of stable power for their system and they pay the price in frustrating troubleshooting and random, power-related system failures.
We chose this 330 W model from Seasonic, the S12-330. It provides ample power and support for SATA drives. Priced at $59, it's economical too. Typically, once the prices drop below $50-60, the quality of choices tends to fade quickly. This power supply represents a choice halfway between the power supplies included in bargain cases and the top of the line power supplies that are overkill for our setup and budget. At the same time we need to underline that it won't be good enough if you plan on upgrading to a fast dual core Athlon 64 X2 processor and a high performance graphics card. Should this be a direction you might upgrade at some point, there is no option to going for at least 400 W.