Final notes: Video card was getting flaky, so I replaced the guts of the box.
Now running a 65W, 3.16 GHz E8500, mobo with SATA-2, PC2-8500 RAM and a fanless GF GT240.
Speed increase is noticable.
Boot time is down to 25 seconds.
hdparm -t /dev/sda: Timing buffered disk reads: 634 MB in 3.00 seconds = 210.99 MB/sec
hdparm -T /dev/sda: Timing cached reads: 14968 MB in 2.00 seconds = 7500.82 MB/sec
hdparm -t /dev/sdb: Timing buffered disk reads: 366 MB in 3.01 seconds = 121.70 MB/sec
hdparm -T /dev/sdb: Timing cached reads: 15054 MB in 2.00 seconds = 7543.87 MB/sec
The stock CPU cooler is very quiet. Total noise is about what it was before.
CPU indicates about 10 ºC warmer than the old 2.4G Northwoood.
Video card shows 20-30 ºC cooler than the old GF6200.
The old parts are not going to waste. One customer has a Pentium 3 machine which was made into a server when it retired from the desktop role, over three years ago. It could benefit from an upgrade. With Linux, old machines live on!