For starters- Yes there is a mac bios. The user end is known as Open Firmware which can be accessed at startup by hold command, option, o, f till you see it on screen. BEWARE-like the programmers button on these machines, do not use it unless you either know exactly what to do, or have a cheat sheet in front of you with commands and info on what they do. Second of all. All G3 iMacs are capable of running OS X. The only thing is that you do the firmware update that is available on apples support site BEFORE installing OS X. I made the mistake of forgetting that at one point and nearly killed the iMac. If you have already made that mistake I have a forum thread on here wher I described in detail how to fix this (can be found at http://www.macmod.com/component/option,com_simpleboard/Itemid,29/func,view/id,2013/catid,19/ ) . Finally ...RAM. To Run OS X 128mb of RAM is required. On machines like that I reccommend at least 256, because other wise, it will be slow as crap on anything below and iMac DV 400mhtz, which by the way you can get away with 192mbs of RAM on but even that will be al little slow. Thats about it I hope you find this info useful.

PS. If you have a drive larger than 7.9 gigs, the first partition (which you MUST install you OS on) must be 7.9gigs or smaller for it to boot.
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Mobile-2.2Ghtz Core 2 Macbook, 4GB ram, 160GB HDD
Server-Dual 1.42 MDD G4, 1GB ram, 4 random hard drives.
PC-C2D 2.4Ghtz, Asus Commando, 2Gb ram, 3 hdd's in raid 5, 8800GTS.