Dual channel will work on an nforce2 board with different module sizes, but you won't get the same performance benefits. The controller will address whatever common memory amount between the two channels in dual mode, and then the extra in the 512MB module in single channel mode. So if you "fill up" the first 512MB out of 768MB total, the next 256MB is only single channel. And keep in mind that the data doesn't get shuffled around to fill up the empty space in the lower memory necessarily, so even if you reduce the memory usage after a time, you may still have data that's being stored in the single-channel section.
I haven't really paid a lot of attention, but I see no reason that ValueSelect or ValueRAM can't be used in an nforce2 board. Just don't try to run it with very aggressive timings. It's still Corsair or Kingston brand modules, made with the same quality, but with chips that can't handle higher speeds and reduced timings.
I personally would recommend just using PC2700 HyperX, rather than trying to get the PC3200 that you may not make use of. The rule for purchasing is get the best you can afford that will satisfy your needs now, not necessarily what you think might be the best for future use. A single module is only 82 dollars for 512MB, and will easily handle some overclocking without having to make the timings too relaxed. Even the 2.5-3-3-6 ValueRAM PC3200 is 94 dollars and would probably end up giving you the same timings at the same speed (underclocking the PC3200).
Also keep in mind that Corsair has a CL2 line of XMS memory, rated for 2-3-3-6, slightly better than the Kingston ValueRAM, that costs only 5 bucks more than the ValueRAM and is more likely to handle overclocking. Even the CL2 XMS PC3200 module is only 95 dollars. Since you aren't going to be trying to run the modules at 400MHz, that would serve just as well as the HyperX. You won't want to try running the HyperX below 2-2-2-5 anyway, and the CL2 XMS will probably handle that just fine at the lower speeds. If you're concerned about the price, that would help you out a bit.