How much will a BIX help

[Enigma422]

I am rebuilding my WCing system and replacing my single fan HC with a dual fan HC. WCing system will be cooling an OCed 4400+ and two 7800GTXs OCed in SLI. My old loop, which cooled a 3500+ and a pair of 6800GTs all OCed consisted of the single fan HC and a BIX. My question is, how much will keeping the BIX in the loop help cooling with a dual fan HC. My main concerns are that the MCT5 coolant I ordered won't be enough to fill my loop with both rads in the loop so I am thinking of taking it out.

Also if I do decide to keep the BIX and I don't have enough coolant, can I supplement the MCT5 with distilled water.

Thanks for the help.