The other folks here are right. Your version of Stuffit Expander is not recent enough to understand the newest .sit format. However, there are other options. You should be able to download the newest Stuffit Expander in .hqx format which the older version does understand. That way you can decompress and install the new version of Stuffit Expander which will enable you to decompress the dnet client. Check here:
Stuffit Expander Download
On this page you need to download the BinHex version which should work fine for you.
A few side notes: No G4 shipped with OS 8.1 installed. In fact, no true G4 will even run OS 8.1 as far as I know. However, it could be an older Power Mac with a G4 processor upgrade. Even if it turns out to be a G3 processor (or older), PowerPC chips are pretty efficient crackers.
GSmith: Though your question may have been a bit rhetorical, yes you can overclock Macs. It is a bit more involved than just going into the BIOS and changing the SoftMenu settings, but it can be done. I overclocked one Mac in particular by doing a little research and then performing some delicate soldering on the processor card in order to change the clock multiplier. Then I increased the bus speed by disabling the onboard crystal oscillator and feeding in a new, faster, signal. I installed a socket for relatively easy changing of the bus speed simply by installing a different speed oscillator. The whole experience was actually quite fun. Tinkering with the electronics on such an intimate level was quite interesting. Also, this was an older Power Mac. More recent Macs can actually be much easier to overclock. Some use standard jumper blocks, which makes overclocking a relative piece of cake.
Anyways, sorry to drift off there for a bit. Hope you get that Mac up and cracking. See you.
Edit: Fixed the link.