This will probably sound like a silly question but...
How much swap space do I really need?
Does it help to have more two or three swap partitions?
Does it help to have one really large swap partition-say 10-20 GB?
I'm probably going to be using a 250 GB hard drive so space won't be an issue.

you don't need much swap really. only enough to cover any shortfall in physical ram. if you have 1gb and don't do massive amounts of memory intensive stuff like video gaming and such, you'll probably be fine with none at all. i would never bother giving more than 1gb of swap to a non-server box. additionally no benefit (from your angle at least) of spreading multiple swap partitions around and such.

I've only got 512MB of RAM. My computer uses RIMBUS so I'm really not trying to increase it. I might use gimp & play games every once in a while

well you won't miss anotyher 512mb i expect, you'll seldom need it though.

The general rule of thumb is to have a swap space 2X the amount of RAM, but generally anything over 1 GB swap is wasted space. So, in your case, with 512mb RAM i would set it at 1GB.

Yeah totally right... if it's still 1992 and RAM costs more than children.

Well, I suppose it depends on how you use your machine. I have 1.5 GB RAM in my system, and I use the swap space sometimes, like now:
-------------------------------------------
tom[~]$ free
total used free shared buffers cached
Mem: 1555388 825912 729476 0 61172 476484
-/+ buffers/cache: 288256 1267132
Swap: 1951888 2432 1949456
tom[~]$
-------------------------------------------------
I do some gaming, video encoding, etc, which can use a lot of RAM. Granted, I never use 1 GB swap, but I would rather have more than not enough, unless I was short on hard drive space. But big hard drives are cheap nowadays.

I've got 2GB RAM and 512MB of swap. It's really a matter of choice, but I think swap shouldn't go past 512MB for a desktop system.