Perhaps it is a G5 iMac? If so, then yes, it will run Leopard just fine no matter the specs. Just ensure that it has enough ram to run smoothly. However, the G5 iMac was replaced by the Intel version in January 2006 which of course would be a far better deal for you

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MacBook Pro 15" | Mac OS X 10.6 • 2 GHz Intel Core Duo • 2 GB DDR2-667 • ATI Radeon Mobility X1600 128 MB • 100 GB Serial-ATA
Hackintosh | Mac OS X 10.5.6 • 3 GHz AMD Athlon 64 X2 6000+ • MSI K9N AM2 • 2 GB DDR2-667 • NVIDIA GeForce 8800 GT 512 MB • ALC888 Audio • 300 GB Serial-ATA
iPhone 3G • 8 GB