need help with new system - ram and mobo mostly

ok, so i am trying to build a good system on a budget, with room to upgrade in the future as my financial situation improves

things i already have:

case: Antec p-180 - http://www.newegg.com/Product/Prod [...] 6811129154
cpu: AMD Athalon 64x2 6000+ - http://www.newegg.com/Product/Prod [...] 6819103773
HDD: Seagate 320gig SATA 3.0Gb/s - no link, sorry
PSU: ThermalTake ToughPower 700W modular - http://www.newegg.com/Product/Prod [...] 6817153039

things i am going to get(known):

video card: ATI HD4870 - whichever one goes on sale for a price i deem worthy at newegg first
optical drive: Samsung 22x DVD Burner w/lightscribe - http://www.newegg.com/Product/Prod [...] 6827151173

things i need help deciding on:

RAM: ddr2 800 or 1066? i know that the processor i am getting won't recieve any benefit from 1066, however i am also looking at future upgrades, and eventually if i get one of amd's quads, the benefit from 1066 will be present. that said, ddr2 800 is cheaper, so it is likely 4 gigs of 800 vs 2 gigs of 1066

after that question, if i get ddr2 800 - 2x2gigs or 4x1gig - the 4x1gig would have better timings, but would it be a noticeable improvement? i don't know anything about overclocking, however i might learn how to once this thing is up and running

Motherboard: AMD 790GX vs 790FX vs 780G vs 790X chipset: part of me wants crossfire capability, even though i wouldn't need a second card for a long while, another part of me wants more than 2 pci slots, in case the 4870 ends up blocking one, i have a TV card i'll be taking from my old computer, and a second slot for my next question. also is the difference between SB600 and SB750 significant?

Sound card - is it worth it? how is onboard sound usually compared to a soundcard such as http://www.newegg.com/Product/Prod [...] 6829102010 - will my 26 dollars give me a noticeable improvement?

thanks in advance for all your help everyone - i've been racking my brain for the past week or two about this, and i can't seem to come to a decision

EDIT:

I am going to be dual-booting Vista Ultimate x64(spend most of my time in vista for gaming) and Ubuntu 8.04 x64

that was one of the 4 or 5 motherboards I had my eye on

another question: as far as the voltage on the ram goes, what advantage does 1.8v have over 2.1v besides power usage? is it faster/slower/etc? what would be good timings to look for on ram meeting your criteria?

thanks again in advance