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.
OK lets do it!
Here is the IFIXIT guide you will need to follow: iMac Intel 20" EMC 2133 and 2210 Video Card Replacement. You'll need to get thermal paste cleaner and fresh thermal paste when you remount the heat sink to the GPU chip. I would stick with the 256MB ATI Radeon HD 2600XT board.
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:
Here's the IFIXIT guide you'll need to follow to upgrade the CPU: iMac Intel 20" EMC 2133 and 2210 CPU Replacement. Again you'll need to get thermal paste cleaner and fresh thermal paste when you remount the heat sink to the CPU chip.
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 Seagate Desktop SSHD. If we look at the spec sheet here: 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.