My server is now running on a Pentium 4 instead of a Pentium 3, with 80-pin ATA instead of 40-pin ATA, and I can really notice the difference! I was first worried about how much extra energy this server would use, but after connecting it to my UPS (with the three hard disks and the DVD writer of course), it looks like it is only a few percent more when not very busy and maybe ten percent more when really busy (like backing up, or remirroring).
Pity for my girlfriend; she'll have to make do with her Pentium 3 for now...