What recommended upload/download speeds should I have for a Quake 1 server?

I can't find the specific numbers for Quake 1 nor the DarkPlaces variant of the engine but for a Quake 2 dedicated server it is recommended to have 2.5-2.8 kbps of bandwidth per player. I would expect that Quake 1 would be around the same or less and will update if I find anything specific to Quake 1.

The Dark Places engine has support for multiple protocols, the default should be DP7 which in comparison to the original Quake network protocol is quite bandwidth efficient but comes with a downside of only other players running Dark Places can connect. You could try enabling this if it isn't already, certainly if you're using the default Quake protocols as these are less bandwidth efficient. The protocol can be changed with the command sv_protocolname.

That said, with the bandwidth you have available I doubt you'd experience any issue in hosting a large Quake server so I would look for other potential causes (not bandwidth related) to the problem you're experiencing.

It's my Wi-Fi dongle. The thing is too slow for anything. I have proved this by comparing download speeds of two computers on my network. One that had my dongle, and another that had a better dongle (I used to use this one, but since I use Linux now, it doesn't work because the linux drivers for it are REALLY bad). My computer downloaded an album I like listen to that is about 454 MB, and it took about an hour. The other computer downloaded it within 10 minutes. Obviously, I haven't checked for upload speed, but I'm pretty sure my awnser is that the other dongle is better, as I have ran servers fine with it before. I'm ordering a new dongle that is better than the best one that I have described, and is compatible with Linux.