Over 4GHz should be no problem with adequate cooling and the right motherboard.
Since the E8500's multiplier is locked upwards (you can't raise it over 9.5) then how high you can take the FSB will play a large part in determining your max overclock.
The X48 officially supports 400 (1600) FSB, and has room to go higher, where the 780i's official spec is 333 (1333). I'm looking into building a new system (the one in my sig has been sold--gotta change that), and was discouraged from getting a 780i by the following article:
I'm probably going to go dual-core, but would like the option of getting a quad-core later, and the apparent 350 MHz wall is way too low for me (and that's with an ASUS ROG board).
DDR3 is too expensive for me, so I'm trying to work out a deal on a Maximus Formula. That lets me get good overclocking while sticking with DDR2. If DDR3 is no problem for you, consider getting a 790i rather than a 780i. Besides the higher FSB and better overclocking headroom, the 790i gives better SLI scalling while using less power, according to Legion Hardware:
(Be sure to look at the scores with SLI, not single cards--they perform basically the same with one card).
At any rate, I'd wait until ATI's new card come out before making a decision. Word is 2 4850s can beat the GTX 280 for less than two-thirds of the price. Don't commit to one platform before official reviews are out.
Personally, I'm reluctant to use SLI, because of all the restrictions and limitations, and I'm not too fond of Nvidia chipsets. If ATI's hardware delivers, I'll go with Crossfire, and if not, then I might just go for one powerful Nvidia single-GPU card.