Righteous OS 10.4 Tiger and 10.5 Leopard retail install disks are in great demand and fetching prices in excess of what many PowerPC Macs are worth on the used market. I doubt you are going to find anyone parting with legitimate system software for cheap. If anyone is, it will be a school or a Mac Users Group ("MUG"). You can use this link to find a MUG in your area:
Beware of gray system install/restore disks; they usually only work on the coupmter with which they shipped.
Also beware of offers to sell you a version of a Mac OS you download and burn to a disk. First, it's illegal software. Second, it seldom works properly--so people have found such versions infected with malware.
eMacs with 700 and 800mHz processors are limited to OS 10.4 Tiger. The faster ones (1Ghz through 1.42Ghz) can use 10.5 Leopard if you max out the RAM.
I have it on a 450 MHz Digital Audio, while not great, it does work.
We've been positing here too long, BD! You anticipated my thoughts that I ended up editing out!
If BD says it works, I trust his wisdom and experience. We may be able to give you more guidance if we knew which of the eMac variants you have (they all are model A1002--there are still hardware differences over time).
As the computer presumably has no OS at this time, you can see the original specs by manually opening the optical drive door to expose a sticker: Example is this Apple article:
Note that, within that article, you can use the serial number to determine your variant. DO NOT post the serial number here, but tell us to which category it belongs---eMac; eMac (ATI Graphics); eMac (USB 2.0); or eMac (2005).
Reasons this helps:
1) It will tell whether your Mac RAM is 1 G or 2G.
2) It will tell if your eMac uses the original (and slow) Airport wireless card ofrthe faster Airport Extreme card.
3) It will tell you us you have USB 1.1 or USB 2.0 ports, important if you what to sync the eMac to an iDevice.
Another thing with which the sticker will help: some early eMacs did not have DVD-reading drives. Most Tiger OS disks and all Leopard disks are DVDs. It would be nice to know if your new find can read the system disks before buying. If the sticker says "combo" or "SuperDrive," it can reader DVDs.