the SB600 is fine for the old phenom aswell, plenty of people got same/similar clocks on both boards. The SB750 was easier to clock, however the power circuitry seems to be much better on the newer boards than on the SB600 based boards, well lots of them, having very hot mosfets under heavy quad load.
You in no way need a SB750, the GX/FX question may or may not be valid. The general concensus always fall down on smaller = better which is the main basis for the question of which is better. Smaller doesn't always mean better though. Older 90nm ath x2's tended to overclock much better than the new 65nm ones. The 65-55nm is also a much smaller drop its a half node rather than full node drop and its new for those chipsets at least and a smaller die is often harder to cool.
I've seen good clocks from both chips though, they seem pretty much as good as each other to be honest. the fact is that, the chipset doesn't even play a hugely vital role, as you can see by the sb600 clocking well, its mostly the chips themselves, the power supply and the cooling.