There are quite a few mainboards with an onBoard Attansic Gigabit Ethernet Chip:

# lspci
[...]
04:00.0 Ethernet controller: Attansic Technology Corp. L1 Gigabit Ethernet Adapter (rev b0)
[...]

The corresponding driver is part of the mainline kernel since 2.6.21 (April 2007), but CentOS 5.x still uses a kernel version (2.6.18) which lacks this feature.

Jay Cliburn did quite a few enhancements on the original driver from Xiong Huang of Attansic and he even built an rpm package for CentOS:

rpm -ivh ftp://ftp.hogchain.net/pub/linux/attansic/atl1/centos5.2/kmod-atl1-2.1.3-1.i686.rpm

Prebuilt rpms for other kernel flavours (x86_64, PAE and xen) can be found on the ATL1 sourceforge project page.