I'm planning on building a budget x58 computer that'll last me for the next 3-4 years. I've been debating on whether or not to get 3 gigs or 6 gigs of RAM. I feel that 6 GB is complete overkill even 5 years down the road.
I'll be using the computer for school/internet/gaming (everything will be stocked, not the type of person that needs GTX 480's in 3 way sli with a 100 inch monitor and everything on super maxed settings with 100x AA/AF).
I'd go for 3GB now. That's usually plenty for most applications/games (not including CAD/HD Video Editing, although it will still suffice). If you find you need more, you can always get more down the road.
I would go for 6 GB, 3 GB may be enough but then you can only expand to 9 GB later on. However, if this is just for schoolinternet/casual gaming an i5 750 is more than sufficient, and then you only need to buy 2 DIMMs of 2 GB memory.
^ You are right, dual channel works in triple channel boards, however, technically speaking, triple channel memory isn't marketing propaganda considering that dual channel offers little performance benefits over single channel, and by advertising 'Dual Channel DDR3 1333 MHz+' really isn't propaganda. However one of the points of triple channel was to offer more memory as well as better performance, so there's no point getting 4 GB with an i7 900, just get an i5 750 instead.