4GB sticks are hard to come by and are usually expensive compared to 2x2GB sticks. I may be wrong; eventually I'd expect the situation that formerly held to be changed.
If you see any good 4GB sticks compatible with your MB for $100-$160/stick or so, by all means, go for it. Few motherboards have 4GB DIMMS listed in their compatibility tables, but this may change as the DIMMS become more commonplace; it doesn't NECESSARILY mean they won't work just because they're not listed in the qualified memory tables, though, it depends on your BIOS / chipset.
Get the best value for a quality 8GB set, it doesn't matter if you get two identical 2x2GB kits or one 4x2GB kit other than that the price / options may vary, and you'll have to be careful to qualify for (or otherwise arrange to do so) the rebates if you buy more than one product per transaction / person / address.
If shopping for DDR2, get PC2-6400 or faster rated memory, with cycle times less than or equal to 5-5-5-15 in the various positions. 4-4-4-12 rated memory or PC2-8000 or PC2-8500 rated memory is excellent, but usually more expensive and it may not be worth it (if so) depending on your needs.
Look for 1.8V-1.9V preferentially to 2.0, 2.1, or 2.2V rated memory.
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