If it has an operating system on it (it might have something like Mac OS 8.5, 8.6, or 9.x), go to the Apple menu on the top left and select Apple System Profiler. Once launched, this will give you all the information you need to know before installing Mac OS X on that iMac. Also remember that you need to update the firmware on it before installing OS X. This firmware MUST be done through a pre-OS X operating system, so don't wipe it clean just yet.
http://docs.info.apple.com/article.html?artnum=86117
You should be able to use a USB keyboard and mouse, but consider that the USB PC keyboard won't have all the keys that a USB Mac keyboard has. I would recommend purchasing a Mac keyboard. The PC mouse is fine as Mac OS X recognizes two-button mouses properly (aka, right click works in OS X).
If you're going to be installing Mac OS X on it, definitely upgrade the memory as high as you can. Also, depending on your iMac you might have to designate the first 8 GB of the hard drive to OS X, otherwise your installation won't work. This means that you have to partition your hard drive with the first 8 GB being the first partition for the OS X installation (it will show up as about 7.5 GB when doing it through Disk Utility). If you need a copy of OS X, check out
Other World Computing in order to purchase a copy of OS X.
While you could get the OEM discs, they don't always support all the Macs since they were meant to be shipped with specific Mac models. The retail version is always your best bet. Also remember that since you don't have a copy of OS 9, you won't be able to use the Classic environment since OS X doesn't come with a copy of OS 9...it's s seperate purchase. Of course, if you have no need to run older Classic apps, then this is a non-issue.
Also, if you're going to get Tiger remember that it only ships in DVD format so if your iMac only has a CD-ROM drive, you won't be able to install it from there. You could use an external DVD-ROM drive and do it though. Or, you could ask Apple for a CD version and they should replace your DVD version with the CD version, but I don't know if they're still doing this.
That's all I can think of at the moment. Hope this helps.