I don't know how to answer your first question. I always use 1 partition so I've never run into problems.
Putting swap on another partition can help, if that partition is on a separate drive. Your HD can only read/write so fast. If you have 2 you can read/write as fast to 2 separate drives. This is like a RAID. You can get faster speed because it's read/writing as fast as it can, but doing it 5 times simultaneously (<-- that's just an example).
So if your swap is on your normal partition and windows needs it, Windows will access that swap. Read/write is the slowest point (okay, second slowest, input is the slowest) in the computing process. Say you're already doing stuff on the computer, the HD is active and all. If you're in a game and things are being swapped you might run into a few slow spots. Not only does the HD have to read/write other things, but now it also has to swap out memory to the HD.
But if the HD is already in use, or already at its peak read/write, simply putting the swap on another partition won't help. It's still on the same drive, the heads are still going around the disc as fast as they can. You can't make 'em go any faster. This means that if the partitions are all on one HD, separating the SWAP partition won't buy you any speed.
However if the swap partition is on a separate HD (if you have 2 or more) then you might speed things up a bit by separating the swap out. That way if the data partition is on "HD 1" and the swap is on "HD 2," then you can still swap things out even if "HD 1" is busy and is already at full speed.
I probably didn't make that very clear. If so, I'm sorry. I'll try to re-word it if I didn't make any sense.