I would upgrade all of the above, If you want buttery smooth gameplay, You mention that it has SDRAM, wich is quite a bit slower than DDR memory. Couple that with a low end video card, and 800x600 with everything turned off will look rotten, but should still be playable. And another area that wasn't covered is the Hard Drive,if it's an older slower model, the levels in DOOM 3 will load like a hamster is powering your rig.
There is a great article about some testing of ATI and Nvidia video cards at [H]ard OCP if you serach for Doom 3 in the review section.
"While we can't be specfic on content, we can share with you some things about the hardware aspects that we encountered. Throughout our entire testing, we did not encounter one visual anomoly. Our DOOM 3 "test2" benchmark ran flawless on all cards tested, including a GeForceFX 5200 series card. This is something that I can't say for some mature benchmarks we run"
I am a member of PimpRig.com, we focus on Overclocking/Modding/Gaming systems.
If you are mainly building this for DOOM 3, I would suggest sticking with NVIDIA, it seems to run better for this game than ATI. If you are considering playing other games too, such as half life 2, ATI will typically run better on games based on that technology.
My current system specs:
AMD Mobile 2500+@ 2.6GHZ (1.83 stock) (88$)
Geil PC4400@520FSB CAS 2.5-3-3-7 (150$)
DFI LAN PARTY Rev B (142$)
Thermalright SP-97 HSF (55$+10$ 92mm fan)
Western Digital 36GIG 10,00RPM Raptor H.D. (110$)
Antec 550 watt True Control Power Supply (115$)
ATI 9600XT (550MHZ Core 700MHZ Memory)(165$)
Pimped/Modified Generic Case 35$
AS-5 Thermal paste 10$
Total Cost: 748$
I think it will play it pretty well, I will be getting the brand new X800 Pro when the price drops a bit (400$now!) and availablity is better.
When using Overclocked settings it will get 15,500 on 3DMARK 01, 4400 on 3DMARK 03,4600 Points on PCMARK 04, and 36,000 in AQUAMARK
Switching to a 9700Pro (A friends) raised my scores to 5400 Points during a quick test, and I am expecting to see over 10,000 points In 3DMARK 03, after I get the X800Pro
The point of my listing the scores, is to give you an idea of what a typical gaming system should get that will run Doom 3 very well.
I looked up systems close to yours on the ORB, and found the highest score to be 8,000 points on 3DMARK 01 (a DX 8.1 Benchmark), when overclocked to the max!A more typical score is around 6,00 points.
When moving to a Direct X 9 benchmark, wich is what Doom 3 uses, the best score listed was 1500 points Overclocked, and around 1,000 Points at stock settings.
Now some hypothetical research:
If you change CPU's to a 2.8GHZ, but still use your Nvidia FX5200, the top score listed is a WHOPPING 2027 points in 3DMARK 03 Overclocked to within an inch of it's life. A more averages score is around 1400 points.
We are looking at gain of around 400 points by upgrading the CPU.
By switching your video card to a regular Nvidia 5900 or ATI 9800, but keeping the 1.8GHZ CPU, the score jumps to 5000-6000 Points!!!!
A 400$ X-800Pro will net you a sweet 10,000 points using a little old 2.4GHZ CPU BTW!!!
For your system, assuming you still want to stay NVIDIA, I would grab the new regular 6800 for 300$ (Check Pricewatch.com) and worry about upgrading the rest of the sytem down the road. They get similar numbers to the X800 By ATI,and support Pixel shader 3.0 (A patch for Far Cry will use it!) wich ATI will not.
I think it's pretty clear that you can have a slow CPU with a fast Video Card, and it will perform 10 times better than a fast CPU and a slow Video Card.
Here is how I would "estimate" playable performance in Doom 3 using 3DMARK 03 scores as a GENERAL guideline, that is the best you can do until it hits the shelves.
Under 1000 points- Slide Show (Unplayable)
1000-3000 Points- Low Resolution no AA or AF
3000-5000 Points- 1024x768 2x AA no AF
5000-7000 Points- 1024x768 4x AA, AF on medium settings
Over 10,000 points, High Resolution, 8x AA, AF on high settings
I hope this guide helps, you can also stop by and see us at PimpRig if you want to get more guidance in your specific parts selection, there are a lot of people with tons of hardware experience, most of us are gamers too!