The amount of Video RAM determines the resolution size your Display can use.
I took this from the product guide:
Graphics memory bandwidth, as assessed by Windows Vista's built in system
assessment too WinSAT.exe, of at least 1,800MB/s at following resolution:
Desktop PC: at a monitor resolution of 1,310,720 pixels (equivalent to 1280
x 1024)
Appropriate graphics memory for specified monitor resolutions expressed as
total pixels (x dimension multiplied by Y dimension):
64 MB of graphics memory to support a single monitor at resolution of 1,310,
720 (equivalent to 1280 x 1024) or less.
128 MB of graphics memory to support a single monitor at resolutions higher
than 1,310, 720 pixels and less than or equal to 2,304,000 pixels
(equivalent to 1920 x 1200)
256 MB of graphics memory to support a single monitor at resolutions higher
than 2,304,000 pixels.
To achieve the memory requirements outlined above on systems with Unified
Memory Architectures (UMA), also called shared memory or integrated
chipsets, 1 GB of dual channel configured system meory (RAM) is required.
The system needs to have at least 512 MB of RAM available for general system
activities after graphics processing.