Ok, now that sounds like a real monitor resolution.
Anyways, given your predicament, I'd say you have a third option besides going for one of the X1900s. There are AGP versions of some of ATIs HD 2xxx and HD 3xxx series cards. I doubt the HD 2xxx series cards would do much good for you because they're hot, power hungry and only come in mid-range flavor for AGP. However, there is an HD Radeon 3850 for AGP bus that would blow the X1900s out of the water. The drawback here though is that most CPUs that are stuck with AGP buses aren't powerful enough to fully utilize the full power of hte 3850. Even so, though, you'd notice an improvement. I'm seeing those cards on NewEgg now for around $150.
Here's all of them
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=40000048+1069609639&Configurator=&Subcategory=48&description=3850&Ntk=&SpeTabStoreType=&Order=BESTMATCH&srchInDesc=
If you can't afford that and instead choose to go with one of the aforementioned cards, then according to Tom's Charts, the X1950XT slightly edges out the X1950 Pro, but that's when they have the same memory. I can't find a comparison for X1950 Pro w/ 512 MB, but I would still go with the X1950 XT anyway unless you plan to get a larger monitor. Any resolution after 1280 x 1024 would probably start hurting a lot from only 256 MB of VRAM.
Edit - Oh sorry, didn't read your post correctly. To answer your question I don't think going from 512 MB to 256 MB of VRAM with the same card will hurt you much. Your resolution should be ok with 256 MB. Still, you should consider the HD 3850 for your AGP bus if you're intent on sticking with that system.