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Yeah, you already have Tiger (OS X 10.4.x) on your system. The last point update to Tiger was 10.4.11 so you could always do that update for free, or you could upgrade to Leopard (10.5.x) or possibly Snow Leopard (10.6.x). Snow Leopard does require your system to have an Intel processor so you will be limited to Leopard if your system has a PowerPC processor (G4, G5).
 
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