Hmm .. there aren't too many mATX motherboards with socket 754. Biostar is a good company, right? Here's theirs.
In fact, here are all four.
Here are the socket 754 athlons: buh
As for the case, I really like my Antec Aria. The power supply has been great - it's powering a hard drive, optical drive, 6800 vanilla overclocked, and a firewire card with no problems.
Dunno what kind of video card you're looking for, but a 6600GT is always a good choice.
socket 754 Biostar motherboard - $60
Aspire X-QPack - $75
2800+ sempron - $75
eVGA 6600GT pci-e - $150
optical, hard drive / misc. expenses ~ $150
You can scale the cost of the video card to meet your budget.