Best icon replacement for a low end Amiga

[Oldsmobile_Mike]

MUI isn't a replacement icon set, however you still need it to run a lot of programs so go ahead and put that on there, along with MagicWB . The only other choices beyond that (NewIcons, GlowIcons, Ken's Icons, etc.) require a more souped up system.

Also don't forget Peter K.'s icon.library!

[Oldsmobile_Mike]

what's wrong with the 3.1 4 color ones?

They're great, if you like your system to look like it's still 1990.  

[Oldsmobile_Mike]

AristIcons. mmm I have never heard of them. What libraries do they use?

A quick search on Aminet found this:
 
http://aminet.net/package/pix/icon/ArtistIcons
 
You'd probably need to install either the NewIcons tools or the Peter K. icon.library to take best advantage of these.
 
Also if you haven't seen it already, this blog has some good tips for sprucing up an old Workbench:
 
http://www.mfilos.com/2012/01/guide-making-workbench-prettier-using.html