my opinion
I looked over your system, and you've already got a lot of money tied up in just the basic system! Plus- PC mag gives it high grades in most areas.
As far as upgrading goes, your mobo is ATX form factor, so virtually any ATX mobo will fit. As I said before, a lot of this depends on how satisfied you are with what you've got. The retail on it that I saw was around $3600. That's a bunch. Upgrading can give you more options on things like gaming, as you can get a mobo with PCIexpress, which is the way things are heading. However, your case, has limited expansion room (drive bays, etc.), so then you might wanna look at a new case. Then.....
In other words, more and more money, on top of your already considerable investment. In short, my opinion, and I am NO expert , is that you should just get another D875PBZ, for a number of reasons, not the least of which is that Windows will not see any significant hardware changes, and the hassles that can follow when it does.
You can find the board around $150 USD, and you'll have your system back again with minimum chaos. You'll be able to run up to 4GB RAM, and geForce has just introduced a 7800 AGP video card, so your gaming wouldn' be that limited. Ordinarily, I'd say upgrade as far as your budget stretches, but you've got so much invested already- I say replace with the same board.
Others around here will undoubtedly differ, but that is my opinion. Good Luck!! :knock: