Well, if you're going with the Quark, I'd say go no higher than 5s. It is supposed to do 6s, but some have had problems with that voltage. 5s doesn't lend it self for even weight distribution though, which is why most buggy people go with even numbered packs, unless you plan on having the batteries on one side.
Gearing: I'm assuming you're using the stock 46T spur? Most go with 14T to 16T (and maybe a little higher, depending on the motor).
Motor: A Neu 1515 2.5d would be good on 5s for about 40mph. A Neu 1515 1y would work on 4s to maintain the 40mph range.
To get an idea of what different gearing, voltage, and motor combos can do, try out this
speed calculator.