Video card remains.
Be sure to apply a new layer of thermal paste to the face of the graphics processor (shown in red).
We have a thermal paste guide that makes reapplying thermal compound simple.
Before reattaching your video card heat sink, use the flat end of a spudger to re-spread the thermal compound over the face of both video RAM chips (shown in green).
If you are upgrading to the 256MB ATI Radeon HD 2600XT (Apple P/N 661-4436), you will need more thermal compound (i.e. K5 Pro) for the two extra RAM chips (shown in green). You can find the GPU upgrade guide here: GPU Upgrade for 20" iMac (mid 2007))
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