You're already at 17k at 2.8ghz. Yes, if you overclock the i7 to 3.6ghz it will probably get close to 20k 3dmarks.
I have a phenom II 940 at 3.6ghz, and crossfired 4770's, and I get 20k 3dmarks. The i7 @ 4.2 ghz with the same cards scores 24k 3dmarks.
Another example is cjl's 4870x2 and i7 scoring 17500 at stock, and 24k with the i7 overclocked. 06 is old now and the i7 seems to take advantage of how it works, and with a high overclock you will get to huge scores.
Actually, it's even more dramatic than that. It's 24k at 3.73GHz (my normal daily use speed), and 27k at 4.2GHz (my benchmarking, Crysis, MATLAB, and Solidworks speed). Of course, that's a pair of 4870x2s, so it's basically completely CPU limited even on the graphics tests in 3dmark06. Still, overclocking the CPU is definitely worth a try.