I've got a 5th printing white box set, plus all the supplements and a 3rd edition Chainmail booklet. It's all in really good condition, light wear on the box obviously... probably between very good and fine for all of the items. Any thoughts on what the whole lot would sell for? Or should I sell them individually? Based on the estimated prices on the site here, I figure I'm looking at roughly $350 or so total. If I package them together, could I get $400?

I think $350 total PLUS is very likely if you sell every item seperately... depends on your auctions bidder action.
(The complete box set being ONE item)

Chainmails 3rd Editions can sell from $25 - $75.
Mint in the SW they have gone for much more...
I have no idea why the huge spread, but any amount between those is possible.
What printing of the 3rd edition is yours?

Other supplements can go for $20-$50, depending on printings and conditions.
If you have all four and in good condition, then $100 total plus is all but a guarantee.
Earlier printings will sell for more... minty first prints can sell for twice as much.

As for the White Box.
I have recently seen a 3rd print (Woodie) sell for just under $500 (Of course it was a BIN and the seller seemingly had no clue as to value).
If yours was in Mint (Box) and Mint (Contents) I could see around $250-$300 if a bidding war happened.
Do not pay attention to the BINs of those sets that are priced at $450 and higher... they wont sell unless to a newbie.

Gnat the Beggar wrote:(Of course it was a BIN and the seller seemingly had no clue as to value)

What does BIN stand for, asking from a NEwbie point of view?  

DOH! I came up with some other good acronyms while thinking about it though.