I have a socket 939 mobo and CPU, so I'll tell you what I do know about this. You could probably search tech-forums, but being as I have nothing else to do right now, here we go:
First off, yes, the multiplier can only go down and not up. I have an AMD Athlon 64 3000+ with a stock speed of 1.8 GHz. The FSB stock speed is 200 MHz. When you overclock you'll want to raise the FSB (I don't know what you do for socket A because I've only overclocked with a socket 939). As you raise the FSB, you want to be positive to keep the HT frequency multiplier so that the FSB x Multi = 1000 or under. If you exceed 1000, you will not be able to boot up. At 200, it should be at 5x by default. As you go up, switch it to 4x until you get to 250, then 3x from there up.
Also, when you raise the FSB, you'll also be raising the RAM speed. You can either try to get as much out of your RAM as possible, or use seperators. If you go into the RAM screen, it should give you these choices: 200, 166, 133, 100. These are proportional to a 200 FSB when determining how the RAM functions so... 200 = a 1:1 ratio, 166 = 5:6 ratio, 133 = 2:3 ratio, and 100 = 1:2 ratio. The goal is to try and get the RAM to work at a 1:1 ratio so there is no bottlenecking, but sometimes you cannot do this when overclocking your system to its maximum point. Sometimes you'll need to adjust your ratio so that your FSB will function at (for example) 270 MHz, and your RAM at 225 MHz (5:6 ratio).
Another thing is that you must lock your AGP frequency. On my motherboard there is a seperate setting to adjust AGP frequency, but many work off of the FSB. If you let the AGP frequency increase with everything, then your computer might not funtion like you want it to.
As a final note, remember to adjust voltages accordingly. If you can keep it cool, you will be fine when raising voltages. Take everything slowly and when Windows boots and loads up, do not raise the voltage anymore.
I hope that pretty much answered your question. I don't know anything about socket As though.
AMD Athlon 64 3000+ socket 939 @ 2.3 GHz
MSI K8N Neo2 Platinum motherboard
EVGA GeForce 6800 GT
2 GB DDR 400 (PC 3200) (2 x 1 GB sticks) (Dual Channel)
DVD player and seperate DVD RW drive
80 GB Western Digital Hard Drive
500 watt power supply
Sound Blaster Audigy
Logitech 5.1 surround sound speakers
SAMSUNG SyncMaster 710T LCD monitor
Aspire X Navigator Case