I distinctly remember being able to do this in older versions (although you might have needed ResEdit for some of them), so this is a giant step backwards, one that seems to have only been corrected in Leopard (and since I deliberately chose a "bionic desk lamp" iMac, and an old enough one to be built as a true 9/X dual-boot, I'd need some sort of "persuader" to take that option). You could also apply a stagger factor.
In Tiger, it appears that the point size of the text controls the horizontal spacing, and the icon size controls the vertical.
With my current screen resolution (1024 x 768), I find that I can get 6 columns of icons if the text size is 11pt or larger, or 7 if it's 10pt. I also get 6 rows with the icon size set in the 64-72 range, but if I go larger than 68, the bottom row gets overlaid by the Dock, obscuring the names and rendering the row more-or-less useless.
If anybody's got a better answer, I'd love to find out about it.