Uhhh, you need both..... badly
Anyone else that responds, how many of you have played the game?
Which do you want more? Without all the feature support of the Radeon(or GeForce) the game looks very poor. Without a decent CPU the game chokes. The Radeon(or GF) supports hardware T&L which will offload your CPU of some tasks, but not enough to "fix" the FPS. The game slows down a 1GHZ GF2Ultra system in some places, to put a perspective on it.
How much RAM do you have? I'll have to recheck it but if I recall it is close to swapping on my rig(320MB).
If you just want the game to
run, then a CPU will likely be the better option(providing you have enough RAM). If you want improved performance(though likely not as much as an extra 400MHZ in the CPU department) and the ability to see the incredible graphics go with a Radeon.
Running a GF DDR and an Athlon 550 right now and turning everything off and dropping to 640x480 16bit is the same speed as everything cranked 800x600 32bit. I've got a Duron waiting for a mobo here(should be today or tomorrow) and can tell you what kind of boost that gives when it gets here.
Oh yeah, what kind of FPS are you seeing?(hit the "t" key and then type fr and hit "enter" if memory serves) This isn't a game to look for 60FPS with, not with anything out right now at least(though it isn't needed either).
Edit-
Just tested a bit. RAM useage, after playing for about five minutes on one of the earlier levels, peaked at just over 270MBs. My FPS didn't change all the way up to 1024x768, and then they plummeted(I would say exceeded my 32MB of VRAM by the looks, dropped from an indicated ~20FPS between 640x480 16bit everything off and 1024x768 32bit everything cranked all the way down to 4-5 indicated FPS at 1152x864(I think, the one between 10x7 and 12x9)). CPU useage was pegged at 100% the entire time(makes sense if I was using AGP texturing at the higher resolutions, it is processor intensive).
I don't think the indicated FPS is correct on this game at the moment, 20FPS looks as smooth as 35-40 in Quake3.