Situation is: Which monitor would be better for gaming?
Dell U2311H (23" 1920x1080 60Hz)or Samsung Syncmaster 953BW (19" 1440x900 75Hz)
the Samsung monitor has lower resolution but I'm wondering if it would be better at
gaming. Would I be able to use Ultra on the Samsung with 4xAA or higher? It has a faster
response time, 2ms, according to manufacturer.
What are your thoughts on the differences in gaming on them?
Response times from the manufacturers are marketing hype, and should be mostly ignored. You could probably get better framerates or use higher detail settings at a lower resolution, but personally I think 1920x1080 with high settings and 2xAA would look better than 1440x900 with ultra settings and 4xAA.
Unfortunately, I can't confirm from experience. It's been over a year since I switched from a 2560x1600 monitor to a 1920x1080 (due to the heat the former generated), and I didn't have any games at the time demanding enough to force me to use high settings instead of ultra even at 2560x1600, so I can't really compare.
Ultra provides more details, but the lower resolution has fewer pixels to show those details with. It's a tradeoff either way - if you can afford a better video card (or a second one for SLI), you might be able to get both the ultra settings and the high resolution.