"How much does dual channel help your computer performace wise?"
Going back to check on the question about how much does dual-channel help, I've found some old Tom's articles that compare things like the nVidia nForce2 with and without dual-channel capabilities. It appears that it's not really that much difference in speed. About the biggest difference in benchmarks seemed to be about 15%. Considering that the datapath is doubled, that hardly seems like a big difference.
My own experience with a Biostar M7NCG-400 (+1.8GHz Athlon) board shows pretty much the same results. Of course, I only used "memtest" from my Linux distro, but it's helped me tweak the performance while making sure that my system remains stable.
As for "matched" RAM kits? I'd suggest just going for matching brands and specs - the one kit I bought had one DIMM that didn't even work. While my own computer is spec'd for DDR333 RAM, I've got DDR400 in it, and it works just fine.
And load it with as much RAM as you can - RAM is ALWAYS faster than disk cache. Even the slowest RAM is much faster. But be sure you match speeds - motherboards will default to the slowest device speeds, so DDR400 mixed with DDR266 gives you the equivalent of DDR266 for the whole memory. Same is true for the timings - two 2.0 chips with one 3.0 gives you the slower 3.0 timing for the whole lot.
The cost of DDR will almost certainly go up in price as it becomes less common (look at the price of old RAM types).
However, I imagine DDR will be around for a while, since there are still huge number of PCs with DDR installed and they won't all be replaced overnight.
I don't know what AMD are doing for their budget architecture - is that moving to DDR2 as well, or staying at DDR? That could make a big difference. For example; s754 boards are still at a reasonable price because AMD is still making chips for them.