Damn roadrunner, 4.2Ghz on the quad??? What was your cooling solution? I see you have your stuff under water, but is it all under 1 loop? What were your temps and what was the Quad at?(Volts)

If you get a 3870x2, I believe you could crossfire that with a 2900XT, just so you know... You would need a new waterblock for the 3870x2 though.

Anyways, If you could overclock the 3870x2 to the speeds you had on the 2900s then there might be a slight improvement, but not by much.