I've been dreading this, but in order to keep from having to do it later, I'm going to go ahead and convert my roms to 7zip. It isn't a required conversion, but seeing as how mame is only going to get larger it only makes sense to use the format with better compression.
Clrmamepro apparently supports an auto conversion, but that program always mutilates my roms.
Over at romcenter's forums I found a batch file posted a few weeks ago by somebody named nayo. It had a few bugs in it (which I corrected) but I've tested it and it converts things properly. You'll need 7zip installed on your system to use this. Just save it to a bat file, place it in your rom folder and run it.
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@echo off
md temp
md done
:LOOP
FOR %%i IN (.\*.zip) DO (SET varx="%%~ni")
IF NOT EXIST %varx%.zip GOTO END
move %varx%.zip .\temp\
cd temp
"c:\Program Files\7-Zip\7z.exe" e %varx%.zip
del %varx%.zip
attrib -r *
attrib +a *
"c:\Program Files\7-Zip\7z.exe" a -mx9 -mmt %varx%.7z *
move %varx%.7z .\..\done\
del * /Q
cd..
GOTO LOOP
:END
rd temp
cd done
move * .\..\
cd..
rd done
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As with anything "blind" like this, I would suggest you make a backup of your roms first, but I have converted a few of mine for testing and verified them to be correct via mame.
A current full romset in mame is around 40gigs, minus the chds. I don't know how much it will save total, but reports seem to indicate that it saves a lot more space then the mame devs estimate.... somewhere around 10 gigs....which isn't a lot in terms of the giant hard drives we have now, but it's a 25% compression boost! There is also a simplier batch file on the site that'll compress your chds, but this seems sketchy to me and I wouldn't reccomend compressing with anything other than chdman (which is flaky atm, so I'm waiting to do the chds until .147).
http://forum.romcenter.com/viewtopic.php?f=2&t=2352