$TTL directive vs. negative cache - 8.2.1

[Auteria Wally Winzer Jr.]

I administer servers that do adserving for a multitude of companies
all over the US and Canada.  Currently 8.2 is in production and seems
to work fine.  My question is the following:

The servers get hit with requests virtually every second or less.  On an
average we can get during heavy peaks over 500 - 1000 hits per second,
which is distributed over 20 servers.  Our image farms will get hit with
the same amount of hits, an additional 10 servers.  Right now I have both
the $TTL and the SOA TTL set for 86400 (both positive and negative).
In your opinion, what should the $TTL represent, since we get hit so often?
I feel leaving the negative cache as is, for it seems to hold up, but I'm
concerned about the overall $TTL.  I was thinking setting it to 3hr, but that
seems to be just a bit too short.

Your opinion is greatly appreciated.  I'm about to upgrade all Linux/Solaris
DNS servers to the official version of 8.2.1, since it fixes many issues with
Solaris (ndc, for example).

Thanks.

- Auteria Wally Winzer Jr.
-- Sr. Systems/Network Administrator
--- MediaPLEX InCorporated
---- 131 Steuart St., Suite 400
----- San Francisco, CA. 94111
------ Email Address: wally.winzer at ChampUSA.COM
------ Email Address: wally.winzer at WahLeeWHIRL.COM