That's interesting...it shouldn't make a doggone bit of difference whether you're running on a P4 thin client or an iMac thin client. Actually, the Power Mac client I was thinking of when I wrote the above was my orange iMac sitting in my living room! It's just about as fast as any other client I have. One of those other thin clients is a Sun Ultra 5 (270MHz CPU), and it, too, is a good performer because of its Fast Ethernet NIC.BTW, Power Macs make pretty good LTSP thin clients. K12LTSP supports this out of the box. So you might not have to throw them away.The one Power Mac works ok -- just sounds like a Kerby vacuum when running. But most of the 60 or 70 machines are old iMacs. They work ok (still no sound due to pulseaudio/liboil "illegal instruction"), but are slow.
The most critical thing you need on the thin client for good general performance (e. g. OpenOffice.org and Firefox, not fancy 3-D graphics stuff like Compiz/Beryl) is a 100Mbps Full Duplex network connection. The server should of course have at least 1Gbps FDX. The built-in video, ATI 3D Rage Pro, will do nicely for that. Full-screen 320x240 video will drop a few frames, yes...but it does work. 640x480 hi-res videos, that might be painful....
The P4 1GB clients I have scream. ;) I'ld like to move to low-powered fan-less thin clients when possible. It would help keep up its "Green Business" status
If you want to do that, then I would suggest Jim McQuillan's thin clients at http://www.disklessworkstations.com.