I am busy with looking for "the right parts" too. It is depending on your budget and wheter you are a AMD / ATI or Nvidia fanboy
I should not choose for a Radeon HD 2**** card, since they are not sufficient anymore and some of them (like the 2900XT) use more power than a Radeon HD 3**** or GeForce 8. And the gurus here say that a GeForce 8 below the 8800 is not a great choice.

Myself, I am still doubting between a GF 8800 GT and a ATI Radeon 3870, when it comes to my own upgrade. It will not be below 512 MB, though!

Maybe the thread about the developments about my upgrade are interesting for you. Mission: Upgrade

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