First, you cannot get a gaming case for 40$. A 50$ case will have no good features. The case is the most original peice of the computer. So your gonna wanna be creative and will have to spend a nice buck.
Yes, if your going AMD definitely get socket 939. The best motherboards for socket 939 is gonna be the
Asus A8V Deluxe VIA Socket 939 ATX Motherboard and
Abit AV8-3rd Eye Via K8T800 Pro.
I would also get better RAM, 512mb 400mhz. Your not gonna get a monitor for 100$. Anything that is half decent. If you want a gaming computer your gonna have to spend some money.
I would also get a new video card. That cards Chipset/Core Speed is only 390mhz and Memory/Effective Speed is 700mhz. I would at least get 400mhz for clock core and at least 800mhz for Memory/Effective Speed . I would go with Western Digital for Hard Drives. You can get 80 GB for around 65$ of course at 7200rpm ATA. Your just gonna have to spend more money. 700$ computer wont be a gaming computer.
Finished With TF.