I had a smallish (4gb) SD card laying around and I was wondering if it's possible or advisable to use it as swap space. Ideally, I'd like to free up some hard drive space by removing the swap on it and solely using the SD card but if that's not possible/advisable I'd be just fine with having a little extra.
All the stuff that can't or shouldn't stay in your RAM is written out to swap and read back in when needed. This means that the swap medium needs to be fast and resilient to lots of writes.
Your SD card fails on both counts. It is slower at reading/writing than a hard drive, and each of its constituent sectors can only be written to a limited number of times before they wear out and can no longer reliably store data.
If you really want to use it to free hard drive space and will be keeping it in your reader at all times, my recommendation would be to consider moving any media files you might have to the card. Media files are usually written once but read many times, and don't need the full transfer speed of a hard drive to play back smoothly in real-time. They are thus the perfect candidates to offload to an SD card.
It does actually help a little if you are getting I/O bottlenecks when swapping since its on a separate device from the hard drive you are running your system on. Although it will drastically reduce the life of your SD card.