specs
evga 780i
evga 9800gtx
4gb corsair dominator
intel q6600 gonna `oc it
wd raptor 150
wd caviar 250
2 samsung dvd burners
antec 900 case
750watt psu
will this rig last me 3 or 4 years?

More than a while, thats a solid rig mate

nothing you can buy now will be good in 3 years, so plan to upgrade a major component every year, and scapping it after a couple of upgrades.

Just some quick suggestions.

Unless you have a desire to go with SLI, I would steer clear of the 780i. nVidia chipsets are always a bit fiddly if you ask me. Not only that but one has to hope that they have truely fixed their SATA corruption issues. I would suggest something from the X38, or P45 chipset. Both of which should be cheaper, and run cooler (especially the P45).

Next, considering recent developments in the GPU arena, I would consider changing the 9800GTX to a HD4850. The 9800GTX should come down in price shortly putting it on par price wise, but it's still subpar IMO performance wise.

Love the case, I bought one for my boys computer. Plenty of cooling, even if you turn the fan switches to low/medium.

Everything else looks great.

Not bad at all. You can add a second 9800GTX in two years, you'll probably need it.

Those SATA corruption issues are more likely to show up if you overclock, so don't overclock until you actually see the CPU used at 100% often.

If you haven't bought already - change one of the burners to an LG or Asus. It's good to have different burners, each is better at some things.

You seem to have the cash, what with the 780i and all. Why not get some modern HDD drives, like the Velociraptor and the WD6400AAKS?