I own a 204B myself and I LOVE it! It still brings a smile to my face when I sit down at my PC. I paid around $500 on mine (I got it very early) and as I have said, I still love it and I certainly think that it was worth it.
Here are the reasons why I would the 204b instead of the 940bf:
The 940bf is a 19" and has 1280x1024 resolution, that is a 'more square' size. It works out to a 5:4, when most standard monitors are 4:3. With things moving towards WS sizes, I'd rather have a 20" that can do WS (with pretty small black bars) then a 19" that is way off.
Since the 19" and 17" LCD's use the
same resolution (again, 1280x1024), the pixels are spaced out further apart on the 19". This can lead to a 'screen door' effect when view up close and generally isn't that great IMHO.
You don't have to run 1600x1200 on the 20". You can always lower your resolution to 1280x960, 1152x864, or 1024x768 if you find that 1600x1200 isn't giving you sufficient frame rates. All of those resolutions are still 4:3 so they won't look bad. I don't know what you could drop down to on a 5:4 monitor and not look distorted.
For the $40 or so price difference, I'd say it's a no brainer. They are both Samsungs which are known to be great LCD's. The way it was explained to me is that you spend more on Monitors, Speakers and Cases knowing that they stick around while all of the other parts change (I add mice to that list). Let's face it, for most of us Mobo's, CPU's and Memory changes happen much more frequently then monitor changes. Buy one that you will be happy with for a longer time.
EDIT: As for how an X1800 XT would do at 1600x1200, check
THIS review out. With mostly every game tested there the X1800 XT kept the min. FPS above 45. The notable exceptions are F.E.A.R. and C.O.D. 2. Please note: a lot of those games kept above 45FPS with 4xAA on, also the test system 'only' used 1GB of Ram. Without knowing your specs. it'd be hard to guess, but I'd think that the review I linked to should put your mind at ease a bit.