I would suggest you go for the larger rad. That rad upgrade would gain you a good deal in lower temps, most of all when under load. The original BIX just isn't big enough to handle a P4 @ 3.9 + 9800Pro, 6800GT or X800XT type vid cards at optimum temps. Most of all if the GPU is also highly O/C'd.

You could however save a good deal on the cost of a larger rad by going with a large heater core.