Well what is it that you want?
As you say yourself: your mainboard can't fit in your current case. You need a new case. If you want good cooling, cases like those from LianLi are the best (though expensive). Cheaper aluminium cases are quite good as well, but not as nice as those from LianLi.
Regular cases can cool quite good as well, but they often need some more fans (or faster (=noisier) fans).
What the mainboard is concerned: what are your wishes? Do you want additional features such as a RAID controller, FireWire etc etc? And what about the CPU? Will you be using your old CPU (is it a socket 423 or 478?) or do you want a new one? Do you want dual channel memory support? Etc etc