I just bought a new graphics card and seem to be having some issues with it. I bought an ATI Radeon x850pro (256 ddr3 / agp8x) and have come across a problem in doom 3. Before babbling, I'll give my specs:

athlon xp2700 @2.167
1024 ddr400
xp home

I just reformatted my pc and installed doom 3 to see what my new card could do. I ran the game with NO AF and I suppose no AA since its not adjustable in the menu. The graphics were set on high by default, so I moved them down to medium at 1027x768. For those who have played the game, my "test spot" was on the last level. With the fps cap off, it ran smoothly over 100 frames at times but stayed around 60-70 normally. When I reached the ending part with the cyberdemon, my frames dropped to as low as 11 and averaged 25-30 which made me upset considering my radeon 9600pro ran that at the same point in the game. Next, I set the game to low quality at an 800x600 resolution. When I made it back to the same part, the frames dropped again into the low teens. After that, and just for the hell of it, I set the game at high quality with a resolution "2 clicks" after 1024x768 (i can't remember) and it ran very well around 60 to 70 frames until the point with the cyberdemon again where they dropped.

I tried another test point in hell in a part with three imps and the frames drop to around 30. The very beginning of the game with the ship floating off the landing pad sits around 30 also. Am I expecting too much? All I want really is the game to run at least a solid 60 frames all the time. I'm beginning to think my cpu has something to do with it considering the frames drop where there are enemies. I realize that pretty effects and more monsters drop frames for most but an x850 should just shrug those things off at such low settings.

I am really confused with why there is no increase in performance at a low setting and low resolution. Could anybody help me out on why this is happening?

If you want to do a 3Dmark03 test and compare it to mine,I'm getting just over 10000 3D marks.This is with no AA/AF and catalyst A.I. set to high.My card is an X800 pro with an A64 3200+.The bit at the start you mentioned,I get in the low to mid twenties with 1024x768 4xAA and 4xAF,the same but with 2xAA in the high thirties.You can adjust the AA in game,it's in the advanced graphics section.Also check out the tweakguide Doom 3.

I don't own a copy of 3dmark03 so I downloaded the demo/trial and scored a 9895. The video tests seemed to run pretty solid but when it got the cpu test, it was very low. There was a test with a couple trolls playing a board game and it was doing 3-5 frames/sec. Maybe its time for me to go with a 64-bit soon

You've answered your own question- it is indeed a CPU bound situation causing this. When you change graphics settings up/down and see no real change, it's almost always a sign of being cpu/system bound.

Doom3 (as well as some parts of Quake4) are notoriously CPU bound. There are spots where the geometry and animations will tank a system like yours down to 5-12 fps. It doesn't matter if you have a 9600 Pro or X1800 in that system, the CPU wont be able to muster anything more.

You can save CPU cycles through various settings though. Sound has a big impact on performance, so make sure you have a good, PCI soundcard as on-boards have been known to adversely effect Doom3 performance. As far as ATI specifics- shadows in Doom3 use a stencil method that is custom designed for NVidia hardware. On ATI's, these shadows can impart a 3x greater performance hit as ATI's are designed for more modern shadows (i.e. vertex shaders vs. old, crappy, stencil buffer shadows). Reducing shadows or disabling can help Doom3 performance greatly as well.

If you search here also- I've got a couple threads with links to the 4.12 OGL file. This reduces CPU load as it only supports older OGL extensions. On lower end CPU's, it can impact framerates as it saves many CPU cycles in OGL games. Do a search for the threads I've posted with that link and instructions on how to copy/override the system OGL per-game with it. Hopefully it will improve your framerates a tad... it's either that or get a faster CPU/mainboard and memory.