512 Mb/1 Gb DDR2 modules - very good overclocking potential. Micron based modules conquered low timings (4-4-4-12) at 1+ Ghz and 1.3+ Ghz with more relaxed timings. May be unreliable though, due to very high voltage required to achieve such good results.
2 Gb DDR2 modules - some overclocking potential, most good modules can achieve +/- 1100 Mhz at 5-5-5-15 or 900 at 4-4-4-12. Some premium modules from vendors like Patriot and OCZ made possible 1200+ Mhz at 6-6-6-18. Usually will work with 2.0-2.1v at maximum stable clocks, which is safe for prolonged use.
4 Gb DDR2 modules - mostly incompatible with Socket 775, overclocking may vary from bad to mediocre, but some can achieve +/- 1 Ghz. Apparently some vendors made 4 Gb PC6400 modules which are in fact compatible with 775, but specs aren't very promising and they are not easy to find.
So as I've said before, if you absolutely need 16 Gb of RAM on your 775 system you should pick DDR3 motherboard with any good and cheap DDR3 memory, like Samsung for example. The cost of a motherboard with cheap DDR3 memory will probably still be less than 4 sticks of mediocre DDR2 4 Gb modules, assuming you can find them. And if you want to get something more XP oriented, you can pick DDR2 1 Gb memory modules with low timings.
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