Paul Robichaux wrote a short article on Exchange archiving tools a couple of years ago. It is still relevant for Exchange 2003, I think. He lists the requirements some companies have for their archiving solution. I think you have to take that list to the different vendors and find the best match. Personally, I do not like archiving solutions that depend on journaling, mostly due to resource use on the Exchange Server. I do like Quest's tools for Exchange management, reporting and archiving.

There are so many options out there for archiving now and more are coming.  Here is a list of vendors who produce or partner with Exchange archiving solutions:

 autonomy.com
emc.com
zantaz.com
meridio.com
educom.on.ca
essential.co.uk
messageone.com
waterfordtechnologies.com
axsone.com
advisormail.net
barracudanetworks.com
postini.com
gfi.com
athenaarchiver.com
liveoffice.com
mailtrust.com
arcmailtech.com
symantec.com
fortiva.com
veritas.com
kvsinc.com
quest.com
sunbelt-software.com
commvault.com
zlti.com
c2c.com
hs-soft.com
ivasoft.biz
mailarchiva.com
exclaimer.com
messagesolution.com
ontrack.com
kroll.com
hexamail.com
sherpasoftware.com
symprex.com
ids.co.za
imrgold.com
ziplip.com
adventnet.com
patronsystems.com
mxlogic.com
jatheon.com
defenderstore.com
kintronics.com
zylab.com
electricmail.com
opentext.com
ixos.com
ca.com
ilumin.com
overtonesoftware.com
gwtools.com
messagingarchitects.com
mirapoint.com
flexnet.com
deployit.dk
PowerFile.com
cryoserver.com
filenet.com
intradyn.com
SDR4.com
simplicato.com
re-soft.com
inboxer.com
email-archiving.net
SonianEmailArchiving.com
Oasys-Software.com
aftermail.com
e-rkive.com
ironmountain.com
mailstore.com
zylab.com
bridgeheadsoftware.com
rad-direct.com
globalrelay.com
amicus.com
PrimeViewUsa.com
brokeragetechnology.com
intermedia.net
kazeon.com

Good luck!