I just bought a shuttle x27D to replace a 2.8GHz Dell Optiplex GX270. The shuttle is a little faster, doesn't play WoW, but I have a real machine for that. I think the thing you miss by staring at benchmarks is the speed of the other old components in your old system. The little 320GB 7200rpm disk I put in the shuttle blows the doors off the 40GB that came in the optiplex. My shuttle with XP boots in about 50 seconds, loads outlook 2007 about 50% faster. I only did stopwatch timing, I don't believe in benchmark software for my own purposes as my goal is real world performance times. Hulu videos hit the processor pretty hard, but I can run full screen 1600*1200 at 480p if I use the hulu app for it. If I run it in a browser window, it chokes on 480p and gets jerky but runs fine in normal mode.
My main goal was saving space as this machine is my girlfriend's kitchen computer. I still can't believe I sold the GX270 for $120 and only spent $220 on my shuttle box and used parts lying around to fill it. Great deal, fast machine. I highly recommend ditching an old P4. Just the savings in power over a few years makes it a worthwhile upgrade.
I added some artic silver under the heat sinks. The power supply brick gets really hot, so i set a sink from a pIII on top of it. Seems to keep the area behind the monitor a little cooler and the cables aren't scary hot back there anymore. The machine itself is just warm. I love it though. I don't get the bashing of dual atoms over an old p4. I'm going to replace another machine as soon as the next gen atom is out.