According to this article click here== both the 6800 ultra and 6800 gt are hit by CPU limitation, even the FX-55.
I can't find any comparison between the FX55 and 4000+ in terms of SLI performance. There is a clock difference of 200MHz. That will give you an increase in performance. No idea how much.
In terms of value for money with currently available CPUs, the article suggests the GT is better value.
If it's between a 4000+ combined with 6800 ultra, and FX-55 combined with 6800 GT, I suspect the latter is a better combo.
On the hard disk, I assume we are talking about SATA? If so, the Maxtor has 16MB buffer plus NCQ (which I assume your motherboard supports). The WD has 8MB buffer (its website claims that its performance matches most 16MB buffer hdds on the market), and no NCQ. I suspect the Maxtor would be a better performer, but simply on the stated specs. Don't know what the actual performance difference is.