Today, you have no option other than Apple branded "stuff." OS X will eventually support third-party stuff, but that is still in its future.
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Which video card is probably not much more than a he-said/she-said kind of thing now that both the Radeon and Geforce are available. The differences between the versions currently supported by Apple are slim. In 6 months that will undoubtedly change as the driver arena matures. Personally, I'm partial to Radeon, but my son believes that Geforce is the only way to go.
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Other than that, personally, I would have second thoughts about a cube.... especially if you are concerened about your video card. Today, the Cube is for people who favor style over function, and who are willing to accept whatever there is that is there. There is a reason that the Cube is cheap.
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If you are concerened about your video card, get a box that has a real PCI slot. And personally, I would opt for the most powerful box I could get. (And today memory is dirt cheap - 512meg should be a minimum for an OS X box.) A fast video card only solves "some of the display issues." Even for "simple gaming" a faster CPU is going to help that card along. But if you want to do iMovie or iDVD renderings ... it will make a big difference.
And then there is still Halo... the OpenGL rendering is done by the CPU, not the video card.
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Personally, I suspect that we will all be looking for new socks (having had them knocked off) after this summer's keynote at MacWorld NYC.
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