That is a load of money for a MoBo. Premium board by current standards, but only for those requiring 3x SLI. Not much point in 3x SLI unless you're pushing 2560x1600 resolutions.
Rather than using three slower cards, most would rather just use two faster ones. Fewer connectors, fewer compatibility issues, less power consumption, etc. Unless you just want to use three cards just because (8800GTX/9800GTX is outdated).
If you're doing a new max spec build, 3x SLI should really only be reserved for 3x GTX280 and GTX260Core216.
Total overkill for 1680x1050 output.
If you don't have all your components today to do your build (when starting with an all-new build), it is always better to wait until you have the money in hand to buy everything at once and immediately do the build. Take advantage of those vendor package deals.
That's barring any special temporary deals since prices still go up after special promos are over.
But holding off on buying will ensure that you don't buy components you can't immediately use before they drop in price. That will save you money.
Card-wise, today you are better off buying the GTX260 Core 216. The difference in performance between that and the GTX280 doesn't justify the price difference, since prices didn't exactly drop when the Core 216 hit vendors.
Case is pretty important since like the PSU, it may well be the only component you end up reusing if you keep it for years. Or you could be one of those guys who has a stack of cases sitting in the closet that are no longer being used and should have been recycled.
If size is an issue (too small), I'd recommend the Antec 1200 for 3x SLI configs. You could probably do just as well with the Antec 900, which is easy to work with and has plenty of space for two dual slot VGA cards with good circulation and cooling, but... the 1200, has even more internal space.
Granted, it doesn't look like a UFO about to take off from your desk (or floor), it still has plenty of pretty spinning LED fans for those who like bling. And you can swap them out for other colors if that's your thing.