....I keep setting my new system build further and further back.
Does anyone have a good recommendation for a mid tower case? ....
"New" is in the vernacular of the writer, I guess. You will be very limited in the MB's available for socket 775. They will likely have been manufactured some time ago and aging in a warehouse. Since such MBs frequently are lined with electrolytic caps ... well, it is your money but yes it can add up quickly.
With the mid-tower identifying the board is not an issue. Any socket 775 micro-ATX or ATX MB should work provided it supports DDR2 RAM.
The slots I was referring to above were expansion slots, not memory slots. Again, with the new case no longer an issue.
The real problem will be finding a new MB as darned few new ones are remaining. Assuming you would like to go 64-bit OS, I'd look for one supporting 4 DDR2 slots for an easy 8Gb. Also, the cost of adding some combination to get 8Gb of DDR2 assuming you are not already running that much may cost as much as adding a 2 x 4Gb DDR3, meaning you could go after [url=http://www.newegg.com/Product/Productcompare.aspx?Submit=ENE&N=100007627%20600007879&IsNodeId=1&bop=And&CompareItemList=280|13-128-536^13-128-536-TS%2C13-157-359^13-157-359-TS&percm=13-128-536%3A%24%24%24%24%24%24%24%3B13-157-337%3A%24%24%24%24%24%24%24%3B13-157-359%3A%24%24%24%24%24%24%24%3B13-157-338%3A%24%24%24%24%24%24%24]a 775 MB that runs DDR3[/url] instead for the same price. Just $aying....Amazon is a good source
of recent discontinued cases at a good price---especially if you are an Amazon Prime customer and get free shipping. Craigslist for local used. eBay for other used.