Apple only offered two graphics cards for this series:
* 128MB ATI Radeon HD 2400XT - Apple P/N 661-4440
* 256MB ATI Radeon HD 2600XT - Apple P/N 661-4436
So you could swap it out. But, before you do that are you sure you want to invest that much here? This is a really old system. Its limited to 6 GB of RAM and doesn't really run very well with the newest versions of OS-X.
Maybe its time to save the pennies to put into a newer system (new or used) instead of putting it into this system.
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=== Update (06/05/2016) ===
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OK lets do it!
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Here is the IFIXIT guide you will need to follow: [https://www.ifixit.com/Guide/iMac+Intel+20-Inch+EMC+2133+and+2210+Video+Card+Replacement/1014|iMac Intel 20" EMC 2133 and 2210 Video Card Replacement]. You'll need to get [https://www.ifixit.com/Store/Parts/Arctic-Silver-ArctiClean/IF179-023|thermal paste cleaner] and fresh [https://www.ifixit.com/Apple-Parts/Arctic-Silver-Thermal-Paste/IF179-010|thermal paste] when you remount the heat sink to the GPU chip. I would stick with the 256MB ATI Radeon HD 2600XT board.
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As to upgrading the CPU I would recommend you do one upgrade at a time making sure the system is still working before moving on here:
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Here's the IFIXIT guide you'll need to follow to upgrade the CPU: [https://www.ifixit.com/Guide/iMac+Intel+20-Inch+EMC+2133+and+2210+CPU+Replacement/28763|iMac Intel 20" EMC 2133 and 2210 CPU Replacement]. Again you'll need to get [https://www.ifixit.com/Store/Parts/Arctic-Silver-ArctiClean/IF179-023|thermal paste cleaner] and fresh [https://www.ifixit.com/Apple-Parts/Arctic-Silver-Thermal-Paste/IF179-010|thermal paste] when you remount the heat sink to the CPU chip.
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The other two upgrades I would do here (I would do them first) is maxing the RAM to 6 GB, I would also upgrade the HD to a SSHD. Since your system is only a SATA II (3.0 GB/s) you'll need to make sure the drive you put in can run at SATA II. I recommend using a [http://www.seagate.com/internal-hard-drives/solid-state-hybrid/desktop-solid-state-hybrid-drive/#features|Seagate Desktop SSHD]. If we look at the spec sheet here: [http://www.seagate.com/www-content/product-content/barracuda-fam/desktop-sshd/en-us/docs/desktop-sshd-data-sheet-ds1788-41602us.pdf|Seagate Desktop SSHD spec sheet]. Looking at the line: ''SATA Transfer Rates Supported (Gb/s)'' we can see your systems SATA port speed is supported. Make sure any drive you put in has SATA II (3.0 Gb/s) listed.
Apple only offered two graphics cards for this series:
* 128MB ATI Radeon HD 2400XT - Apple P/N 661-4440
* 256MB ATI Radeon HD 2600XT - Apple P/N 661-4436
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So you could swap it out but before you do that are you sure you want to invest that much here? This is a really old system. Its limited to 6 GB of RAM and doesn't really run very well with the newest versions of OS-X.
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So you could swap it out. But, before you do that are you sure you want to invest that much here? This is a really old system. Its limited to 6 GB of RAM and doesn't really run very well with the newest versions of OS-X.
Maybe its time to save the pennies to put into a newer system (new or used) instead of putting it into this system.
Apple only offered two graphics cards for this series:
* 128MB ATI Radeon HD 2400XT - Apple P/N 661-4440
* 256MB ATI Radeon HD 2600XT - Apple P/N 661-4436
So you could swap it out but before you do that are you sure you want to invest that much here? This is a really old system. Its limited to 6 GB of RAM and doesn't really run very well with the newest versions of OS-X.
Maybe its time to save the pennies to put into a newer system (new or used) instead of putting it into this system.