6800 > 9800xt but since it has 128mb of ram and it's not GDDR 3 1000mhz + ram, it runs slow at 1600x1200 with Antialiasing and Anisoptropic filtering enabled.
In my opinion x800pro is not the best value since you have to unlock the extra pipelines to truly turn it into x800xt. Since this requires a hard mod (physical changes to the board) and only x800pro vivo cards can do it, and nothing is guaranteed, I woudnt recommend this option for you.
6800GT > x800pro anyways for about $370 on pricewatch.com
Since 6800Gt has 16 full pipelines, overclocking it aids a larger improvement than overclocking X800pro due to its 12 pipelines.
If I was you, i'd sell 9800Xt and get a 6800Gt and then overclock that to 6800Ultra speeds. If you are really an ATI guy, just sell 9800xt and get X800xt.
I wouldnt upgrade to 6800. But definately try to sell 9800xt, even if you sell it for $150, it's still not bad and goes towards the upgrade.
Also if you only play at low resolutions like 1024x768 and without quality features enabled like AA/AF, i wouldnt get anything faster than 9800xt you have. Upgrade only if you prefer to have eye candy. 9800xt will perfectly handle doom 3 and half-life 2 at 1024x768 HIGH settings.
here are some reviews that might be of interest to you:
Graphics Buying Guide - TomsHardware - Nov 10th, 2004
VGA charts 4
October 2004 Digest
Anandtech Roundup
The Fastest Graphics Cards of Summer 2004
As far as the 2 games you are concerned about here are specifics:
Doom 3
HL2 (Beta)