Spending $250 on a X1950XT is not worth it. Get a 7900GT or X1950 Pro, but don't go for the Gecube, it's not worth it. AGP is in the autumn of its life. Don't spend a ton on the best AGP card, you won't see the gain over a performance-range card like the previous 2 mentioned.
Check the VGA charts. Framerates in games are good real-world numbers. If the extra XX fps really will increase your enjoyment, or if it means you can turn on more eye candy, than go for it. I would say go for a x1650 Pro (specifically this one):
Your antec will handle the 1950 pro ok couldnt say for sure about a 1950xt as im not to familiar with psu brands and how good they are but i think its one of the better ones.
The 1950gt is the same core ie same card as a 1950 pro just with slower clocks so in theory could be oced to about the same speeds.
1650pro forget it i have a rig with a 1650xt which is capable of playing oblivion at mid+ settings at about 35fps average(cpu similar to you ) and have seen reviews that say the pro isnt in the same league
The 7600gt is about equal to the 1650xt
As has been said the 1950XT. isnt worth the extra cash and the sensable way to go is with the 1950 pro its relativly cheap and is a powerful card.
The choice between 256 or 512 really comes down to things like do you play at high resolutions or will you be playing games with high texture counts if so then the 512 would benefit you other wise the 256 will be fine.
I also have an Asus P4C800 E Deluxe mobo with Northwood P4 3.4.
Have to replace my burned out X800XT-PE. Was told that anything "above" an X1950 PRO would be a waste of $ cause the cpu would limit it. Just ordered this ................ http://www.newegg.com/Product/Product.aspx?Item=N82E168...