As for the GPU, something like this should be acceptable:
http://www.newegg.ca/Product/Product...82E16814187030
Overclocking isn't that hard, but if you have a brand-name computer it may not be possible. Look up AMD Overclocking Tutorials for some information about it.
Also, you may want to install a 64bit Operating System, I believe Dolphin 64bit builds are a tad faster.
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