You don't need a sound card with a Pegasos! 16bit sound support is built in! As is network support.
Memory is a problem. You can't just get it from a PC hardware shop and expect it to work - most wont. I got my DIMM from kdh-datentechnik in Germany - a single 512MB one, PC133 registered buffered CL3, Kingston brand. Works perfectly. See the compatible memory list on MorphZone to pick DIMMS - and to see what ones to avoid.
You'll also need UDMA cables for the hard drive - the 80pin ones. KDH also sell these. You might need one for the CD-ROM too. Don't put the HD and CD-ROM on the same IDE channel or you'll slow the HD down.
Try to keep to PS/2 mouse and keyboard for now, and avoid USB. MOS currently needs PS/2 devices to boot, but this will change in the future.
In fact, I got my whole order from kdh - including the keyboard, which was a mistake on my part because it ended up being a German one! QWERTZ is really quite annoying. ;-)
Edit: A nullmodem cable is also a must-buy, so you can diagnose problems if your Pegasos fails to boot. Which kind depends on the serial port of the machine you want to read debug output on. The Pegasos's serial port is 9 pin male, same as most PCs. An amiga is 25pin male. I have a laplink one that has both sizes on each end.