I tested both and there is no question CoD2 in DX7 mode runs smoother. Seriously.
It dumbs the graphics down somewhere between cod1 and the DX9 settings. It's actually easier to see the enemy with less shadows which makes MP better imho. You still get the the ambient smoke and nade smoke, just less depth in the textures and less shadows.
However, I ran the entire SP in DX9 just for the eye candy but that is with 2gb of ram and a 6800. That is a straight-up BFG6800 with 128mb.
here is a tweak guide that may help gain a few fps but don't expect much.
http://www.tweakguides.com/COD2_1.html