Help! New gaming build ram/mobo

Hi, I have around 900$ budget for this gaming rig, the first part, I have no problem choosing cpu/video card but I don't know how to chose mobo+ram

here are what I chose for begining, the pieces are avaible at my local store, I won't be buying from internet because shipping is very expensive in Canada, bold writing are those which I am not sure

Intel C2D E8400
Sapphire HD4870 512mb
OCZ GameXStream 600W
Antec Nine hundred
WD Caviar SATA 320GB
Asus P5Q-E <I don't know any other mobo than Asus so I chose the one not too expensive, 2X PCIE2, I would like to crossfire later

RAM
OCZ DDR2 PC2-8500 Fatal1ty Edition 2GB 60$
2.1V 1066MHhz 5-5-5-15

or

OCZ DDR2 PC2-6400 Platinum 2GB 60$
2.2V 800Mhz 4-4-4-15

or

Corsair TWIN2X2048-6400C4 60$
2.1V 800Mhz 4-4-4-15

Which one is the fastest?? Please suggest if you have better option for around the same price ^^

I have no idea how to choose right ram memory, 2gb will be enough for me but my motherboard support 800Mhz/1066Mhz/1333Mhz, here's the problem, will it be better to choose the highest frequency that mobo supports with higher latency or lower frequency with lower latency

Thanks