I guess apart from the Ballistix, they arent aware of any 1GB modules that overclock well , hence the refrain from selecting 1GB sticks.
True 4 DIMMs populated reduces your overclock, but I guess you could remove 2 sticks and leave just 2 sticks in and OC.
Enermax is a good enough choice IMO. Something better than that would be the PC P&C. OCZ would be suspect - after its recent fall at the THG PSU testing.