A new video card may help equally as well as a CPU would, but you probably can't upgrade the CPU much. Unless you can upgrade the mobo too I would just go with a nice graphics card. One that's better than your system, because then when you do finally upgrade your computer the graphics card can just be ported over. Don't get the Ti4200 if you live in the US, that thing is $170. Get the $99 ATi at newegg (heh I've said this to like 6 people already). This ATi card can <i>consistently</i> clock at 300/300 because it uses 3.3ns memory. That's as fast as a GF4 Ti4400 and it has better anisotropic filtering too (less performance impact when turned on). Seriously I don't see any reason why you should get the Ti4200 when the 64MB OEM card at newegg comes with 3.3ns RAM. You can OC that sucka to be better than a Ti4200 and it's $70 cheaper.
Course if you don't care about price at all, get the Ti4600. But don't get the Ti4400, because it doesn't fit into any buying plan.
Unreal 2 is supposed to be CPU dependent, as are most D3D games, so a new Gfx card won't allow you to do much with just a 900Mhz Athlon. Morrowind just runs slow on practically any system, so the best you can hope for on your system even using the best video card out there is just running the game on low low low detail settings (not at the lowest but pretty close) and 640x480.
Like I was saying earlier, your best bet is to upgrade to the $99 OEM ATi 8500 for $99 with the 3.3ns memory (read the user comments to make sure it's the right one) right now or to get the top of the line one (either the Ti 4600 right now or wait for the Parhelia, R300, NV30, P10) and then re-use that card with the next CPU/mobo upgrade you do. If you do that, your buying pattern will be like this.
Buy CPU/mobo/memory
-wait one year
Buy high quality gfx card
-wait one year
Buy cpu/mobo/memory
-wait one year
Buy high quality gfx card
and the cycle repeats. This is currently what I do, and it works fine. There's at most a 6 month window when I can't run the newest games at the highest detail settings.
Censorship makes us so much more creative.