If you live anywhere near a Fry's Electronics, watch their ads. They routinely run special combos that include a motherboard and an AMD CPU, such as a Sempron 2500+ for around $80. Most of the motherboards they include are ECS models, and some of them aren't worth the hassle, but others use an nVidia or VIA chipset, and they make excellent basic systems. The better boards include onboard NIC and at least minimal sound which you can upgrade as you want, later. They also regularly run specials on name brand RAM (Kinston, PNY, etc.), such as 512 MB for $50 - $60 after rebate.
If you're serious about gaming, you'll want more than a basic vid card, but since the price of the CPU, motherboard and RAM are well under your budget, you'll be able to afford a little more for that. 😎