I really don't what know what your're asking, but here's what I know:
Some ASUS boards, like mine, can preserve dual channel mode if you add in another single channel stick. My board also has three slots, as with all nForce2 chipsets do I believe. And no, you don't really have to worry about timings, they don't have to be identical.
Depending what RAM you already have and what RAM you're adding, you may or may not notice a difference. Chances are you will, if your RAM models are similar.
ASUS A7N8X-E Deluxe
AMD Athlon XP 3200+
OCZ 1GB Platinum PC3200 Rev2 Dual Channel
ATI Radeon 9800 Pro
Enermax Noisetaker 485W
2 x 80GB SATA Maxtor Diamondmax 10 on RAID 0