I would approach the issue from the other direction. Start by buying enough RAM to contain the active working set for the foreseeable future. Afterward, you can start experimenting with different swap sizes (swapfiles are easier to resize, and they perform exactly as well as swap partitions!) to see how many inactive anonymous pages you can safely swap out. If you can swap out several gigabytes, that's a bonus! But don't take that for granted. Always be prepared to move everything back into RAM when needed.