Look beneath the brand name and do some math. Most manufaturers of DIMMs list the speed of the individual chips on their modules. They vary from:
5.0ns = 200 Mhz
6.0ns = 166 Mhz
6.5ns = 150 Mhz
7.0ns = 143 Mhz
7.5ns = 133 Mhz
That does not mean that the manufacturer can not test for better performance than the chips specify, but I'd put my money on the module that uses chips with the lowest number to start with. According to Crucial, their modules use 7.5ns chips. Kingmax 256MB PC2700 modules use Tiny BGA (5.0ns) chips. Unfortunately, I have not seen any 512MB modules with Tiny BGA 5.0ns chips yet, so if you want a single module solution that yields 512MB, then your choices go down. It's probably just a matter of time before Tiny BGA (5 or 6ns) 512MB modules will be available though.
73, Hoot