I don't think the zalman vga block would be restrictive at all. It's a simple, single-pass tube with no pins, fins, grooves, or anything else to promote turbulence and good heat transfer. IMHO, that VGA block is for quiet, just-barely-enough VGA cooling when using a low-pressure pump like in the zalman reserator system; i.e. performance users look elsewhere. It's a way to eliminate the fan on a gpu card for noise reasons.
This type block would be a decent choice for a chipset block in many watercooling systems, but chipsets don't put out that much heat either. Compare the construction of the Swiftech MCW-20 chipset block vs. the MCW-50 gpu block. The MCW-20 is a simple pass-thru chamber with no restrictions. The MCW-50 has a diamond pin-fin matrix for greater heat transfer.
Considering the rest of your setup, I'd stick with the current GPU air cooling or get a good GPU waterblock like the MCW-50 or better.
