When designing a Vista start panel, make the left side wider than the right because the all programs list pops up in the start menu itself instead of a fly out like xp. Long names get chopped off if your left side is to narrow.
For Vista: Using a start menu that is 380 px wide, I make the left side panels 230 px (both the Programs and All Programs panels), and I make the right side panel 150 px wide (the Places panel).
For XP: Using a start menu that is 380 px wide, I make both the left and right side panels 190 px wide.
For me, these sizes work well...no text gets clipped off, or gets split into 2 lines of text.
Hope this helps....