Modern pizza box servers can pack a decent punch as well when compared to conventional servers.
Dual processor machines are quite common, as are slots to install up to around 2 GB of RAM. Usually you are limited to three physical disks (either SCSI or SATA), but with 250 GB SATA disks being decently priced, you can get 500 GB of RAID 5 disk space.
Most have dual onboard NICs, and two PCI slots, so you could have up to 10 network interfaces (on the basis that the most ports I've seen on a single PCI card is 4). And most can have two PSUs installed.
So yes, they're not at the high level of power, storage or fault tolerance of a conventionally sized server. But they do provide enough of all of these for most applications.