All new platforms (AMD's AM2 and Intel's 775) use DDR2 now. Although the difference in speed in MHz looks big compared to "old" DDR, the overall performance is quite the same, due to bigger latency.
Dual channel use two physical sticks of memory (2x512MB or 2x1GB and so on) which greatly improves read speed from memory (almost twice). So yes, 2x1GB = 2GB, but you need to have two free memory slots to insert them :) .The idea is similar to this used with hard disks and RAID 0. Generally speaking, to work in dual channel flawlessly both stick's should be from the same manufacturer, type, size, series etc. that's the reason why they are sold already paired. Of course you can try run two sticks not dedicated to dual channel in dual channel byt there are no guarantees that they will work.
About the performance. I made some tests with Lock On 1.12 and custom track I've made and the results are: (tests are with DDR, but DDR2 acts very similar)
Dual Channel:
2x512 without dual channel: 29.51 fps
2x512 with dual channel: 30.80 fps
Memory latency:
3-4-4-8-2T: 29.14 fps
2-3-2-6-1T: 30.80 fps
CPU speed (AMD 64):
1800MHz : 23.74 fps
2400MHz : 29.95 fps
2750MHz : 34.71 fps
So if you are short with money, better buy fast CPU and 2GB of cheap memory. :smilewink:
Ah, and remember, that due to technical limitations Windows XP 32 bit won't benefit fully from 4GB of memory. You'll need to buy Vista or Server 2003, or XP_64