There are no drivers for the 2X00 series cards yet, one is promised "before the end of the year", see articles here. Edit: Make that this coming...
Unless you're going to overclock the CPU, I don't think you'll get any benefit from ddr2 800 over ddr2 667.
The ubuntu live/installation CD will set up GRUB for you, so that shouldn't be a problem. I have been told, though I've no experience of it...
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