I thought DDR2 was actually going up in price as production stops, regardless, I'll try to help.
1. It depends on what you do. I list of applications/tasks you do would help. Any specific games you play a lot also.
In general, most apps are 32-bit and coded to only use ~3GB anyhow, but some apps do take advantage of more.
http://gallery.live.com/...f421da&pl=1&bt=1
Keep it visible on your sidebar while using your computer as you typically do, if you see your ram go below ~200mb free, then you do need more.
2. Probably, the memory controller is actually off-die, so it depends on your NB. You may have to drop down your latencies.
3. Yes and no, depending. More channels means more raw throughput (think of raid), but you may have to drop latencies (as above), and in some cases it can drop performance regardless. It shouldn't really matter.
On a more general note, more ram isn't going to matter unless you're running out of it. Even then, most things are limited more by CPU/GPU than ram speed.
If you're looking to speed up your system and determine you have enough ram, you could invest in a heavier CPU cooler and try to increase your overclock.
If you could provide a more detailed set of system specs (mobo, GPU, hard drives) along with what applications you use, it would really help me figure out what would be best.