strictly depends on overclocking really.
there isn't a CPU or mobo out there that's default is to run at PC3500 speed (433Mhz) so it is strictly to do with overclocking for the most part.
truthfully, Id be tempted to go with the gig of 2x512 PC3200 if it wasn't going to be overclocking.
the Hyper-X is great memory, will also run at lower latencies, but having a gig, if you are not overclocking your PC, is what i would do, probably.
keep in mind, you could be using an older CPU or even a Barton at 333 FSB, and still not be overclocking the PC3200, so there is still a bit of headroom for the PC3200 RAM in some cases for overclocking... you WOULD be fine though with 512 of Hyper-X.. 512 is "enough" thesedays, but in some cases, you can actually see where a gig makes a difference.. (some big games)