Hi Sleepy One, welcome! #4 is moot, since a 15" laptop definitely falls under the "desktop replacement" category. I see Dell and Sager meeting your needs. For Dell, you can get a Studio 15 (starts at $649), then upgrade the card to that 256MB ATI.
Sager offers more value. Get the NP7682. You'll have to spend a bit more at $769, but that gets a much more powerful video card.