cjl :
You're CPU limited. My 4870x2 quadfire setup only scores 17k with a stock clocked i7 965. At 4.2GHz, I get 27k without touching the graphics cards.
No offense but that is not true, neither of you should worry about synthetic, only actual performance. A Phenom II 940 @ 3.6 Ghz will do just fine.
As for 3 4890s, they are quite good. I have 2 Sapphire Toxic editions running right now at 1095 Mhz core / 1124 Mhz memory and I'm still trying for higher. Should you get a third? F&%k no! 2 4890s will do quite fine and the third is just for bragging rights. I bought them for the sole purpose of seeing how 3 maxed 4890s compare to 3 maxed GTX 285s, 2 of them max Crysis without any help from the third.