I'm using an old pc with only 64 MB of RAM since that's all the motherboard can use. I have both kernel 2.4.32 & 2.6.17.3 on it and have noticed with top that 0 k is used by swap when it starts. After starting an X session and exiting X, top says that used swap is 1.5 M with kernel 2.4.32 and 4 M with kernel 2.6.17.3. Does swap go back down to 0 k after some time? How could I free up some memory with Slack 10.2?

EDIT: I use this pc as an mp3 server with gnump3d.

Let the kernel manage the memory. Actually, you can run flxubox, or even window maker with only 32MB of total memory. I've run with 16MB physical RAM and 16MB swap, if you run opera, after about 3 pages in the cache it will crash.
So, with 64MB you have a little that can be freed up by having some swap space around. It can be a partiton or a swap file. For smaller sizes or single-partition systems, I use a swapfile in /root. Even a 512K swap should help keep the kernel from doing anything weird. But, for having a decent working environment for browsing or graphics editing, etc, you'll need to add MB's or GB's as needed.

Note that the minimum size for swap is just 4K. I have systems that have run for a long time without any swap at all. Only recently was I able to make one hang-up -while compiling the full mozilla with tests and debug stuff on a machine with around 200MB RAM. It would choke and lock up after about 2 hours on the job. That was on an iMac, though, which be the reason.
Sounds like you would be fine with 16-64MB of swap, but if you have the space give it more.
For normal use on PI & PII machines, I don't notice any improvements in performance above 258MB of RAM.
Also, some of the fastest RAM available is from using 32MB sticks! Sometimes, you can actually get better performance by using a 32MB stick in the primary slot, or two if you have 4 slots.

Another trick for improving the performance of old PI's and PI'S is to use a kernel compiled for i386 instead of i486 or above, and get the size of the kernel down to under 800K.