Or maybe someone could recommend a different card? It must be PCI or PCIe 1x
You should, generally, avoid straight PCI cards. They're expensive, slow and poorly supported. PCIe 1 is a better slot, although not good.
Will cards built for DirectX 10 and shader model 4 work with morrowind?
Yes, Morrowind requires DX 8 and SM 1.1, at minimum.
Microcenter is generally a good store, I get most of my stuff from them. I'm not familiar with nVidia cards, but you want to go with a mid-range card from a good manufacturer. I've heard of EVGA before, not sure how good they are though.