IMO, go for the 2500K and a pair of 6950s, with a pair of 6970/580 and a 2600K, you get ~20% performance increase but with a noticeably flatter wallet. You would even minimize the 20% with an healthy OC on both. A pair of 1Ghz 6950s and a 4.5Ghz should be very close to a equally maxed out 2600K/580 setup. Besides, when the 20% starts to make a difference, the more powerful solution is probably outdated also.
Save the money and update a little more often, so you can have some flexibility. You can have more fun with tweaking also.