I think you're misunderstanding the labelling of RAM.
DDR2-800 (PC2-6400) runs at 800MHz while DDR2-1066 (PC2-8500) runs at 1066MHz.
The PC2-6400 refers to the data movement per second of the RAM. Ie, PC2-6400 can move 6.4GB/second. So in theory, PC2-8500 will be alot faster than PC2-6400. In reality, PC2-8500 usually has poorer latency times (the delay from cpu instruction to RAM operation. You could think of it like a car with a bigger engine that has a faster top speed but takes longer to accelerate.
PC2-8500 RAM is also alot more expensive than PC2-6400 RAM and coupled with the looser latencies it makes PC2-8500 only for enthusiasts. I'd recommend you go for DDR800 until DDR1066 becomes more mainstream.
Oh, and you can use DDR800 RAM with a FSB of 1066, you'll just lose the performance benefit of having the RAM running at 1066.
Well, I have the Corsair 8500, works great. The 2GB cost me a fortune, but it is rock solid. I run Vista Beta and Oblivion. Fabulous stuff.
Stays nice and cool too.
I have no idea about Kingston. But I know them to be solid reputation wise.
Honestly though, unless you are running a CPU 1066 FSB, it is not worth it to buy either.
What motherboard are you using? Some motherboards don't work well or work at all with certain brands. Most common problems are when people use OCZ in an Asus board - those two don't play nice together.
If you have an Asus, I would recommend you stick with Corsair/XMS and Crucial/Ballistix. My friend has an older (478, I think) Asus board with Kingston value, and I hear it works well. But I don't really know all that much about Kingston, especially not its HyperX line.
If you want to overclock to extreme heights, by all means get DDR2-1066 (PC-8500). But for normal use, you'll be better off with PC-8000 and a better video card.
Yes, I've had a look through the P5B deluxe specs and no where does it say it supports crossfire (which is wierd since ASUS would stamp it on your forhead if they could). It's well known that the intel 965x chipset has some problems supporting dual graphics solutions. You might have to go with the 975 chipset to get full crossfire support and then you run the risk of not being able to use conroe.
PS, is it just me or has the ASUS features page exploded in size, it's about double the size of what my A8N-SLI Premium had. I'm not complaining though, AI Gear and Crash BIOS 3 look like some pretty snazzy additions.