Both if you can. I would stay away from the CII, you have a slot type board? Find a slot 650/700 PIII, they are becomming cheaper on the used market as well as new, and in about every case will best the CII 566@850MHz.
Find a Geforce1 DDR used for a little more than you can find a new MX.
Even the earlier DDR board will beat the MX in every case, especially once you oveclock the DDR ram.
I'd buy the vid card first. I've been in a situation you're in awhile back, and went the vid card route. However I think the TNT1 would need at least a 550MHz CPU to peg it (as far as QIII fps are concerned).
I used to get about 45fps 800x600 16bit all sliders all the way up, high quality snd on, sky off, brass off QIII time demo 1 with my old 366@550MHz, TNT1 card clocked 110/125mem. Got like 75fps 1024x768 whith a simple switch to a DDR Geforce1