iMac 5,1 Hardware Upgrades

What hardware upgrades can be done to the iMac 5,1 ?

Hardware Overview:

Model Name: iMac

Model Identifier: iMac5,1

Processor Name: Intel Core 2 Duo

Processor Speed: 2 GHz

Number Of Processors: 1

Total Number Of Cores: 2

L2 Cache: 4 MB

Memory: 4 GB

Bus Speed: 667 MHz

Boot ROM Version: IM51.0090.B09

SMC Version (system): 1.8f2

Serial Number (system): W8******VUX

Hardware UUID: 00000000-0000-1000-8000-0017F2D1F022

I know it is possible to upgrade the Hard drive.

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