Re: My NEW RIG!!!
THe main thing with SLi is the cross GPU support. Games are slowly in the process of being coded to take advantage of this technology and the more RAM and higher clocks, the better. It may seem a little "overkill" right now or for a majority of things that are out, but in a year, that baby will become mainstream. So I would get whatever you can afford. If it was me right now, at this very moment in which I am in a building process, I would buy a single 7800GTX 256MB and wait to get more money to buy a second one. I don't have that kind of money where I can pay for the 512MB version right away. Plus, you get shared GPU support that way so you're bound to have the same or better setup than having a single 7800GTX 512MB. Unless you plan on SLi'ing the 512MB version. EXPENSIVE.
SLi x16 is standard for graphics port and you really can't avoid it. All new motherboards with the nforce4 support should have it and it's becoming the industry standard now. You must have it for performance. It will get better and better in the future.
*Fact: Microsoft Window's Blue Screen of Death vs Computerforums.org's White Screen of Death. Which is worse?