Funny, I built a Dual Opteron 64/248 box with 2GB memory, and it eats G5's for lunch. Any day, Every day. It also is running Fedora Core 3, with the 2.6.9-1.latest SMP kernel. It costs anout 1/4 of the equivelent Mac box.
I take it you're not talking about a rack-mount system you built. A dual Opteron 248 1U costs about the same or more than an XServe, depending on options and brand. It would still be faster though.