Generally speaking, the parameters are how much system resources you have. If you have more than 256MB of system ram (and a high percentage of that is free), then using the larger texture format may not be an issue.
But, if your video card only has 64MB of ram, then the larger texture format could be an issue.
If you have 256MB of system ram and 128MB of video ram, it will depend on the amount of available free system memory.
The mixes and matching to give an accurate answer are rather staggering.