My plan is to get a Core Duo 6300 and this motherboard. This board takes your existing AGP card but also has a PCI-E slot, and it also takes existing DDR memory as well as providing DDR2 slots to upgrade later -- gets good reviews as an intermediate measure and it's cheap too.
I'll upgrade to a PCI-E card when I'm ready, upgrade the RAM to DDR2 when when I'm ready, then finally go for a full PCI-E motherboard at my leisure. (There are minor speed limitations on the PCI-E and DDR2 memory, but it seems like a great upgrade path to Core Duo/PCI-E if you're on a budget or don't want to fully commit yet.)
I'll upgrade to a PCI-E card when I'm ready, upgrade the RAM to DDR2 when when I'm ready, then finally go for a full PCI-E motherboard at my leisure. (There are minor speed limitations on the PCI-E and DDR2 memory, but it seems like a great upgrade path to Core Duo/PCI-E if you're on a budget or don't want to fully commit yet.)