If you're a big fan of upgrading I'd say to go with a AM3 mobo and DDR3 RAM, possibly a USB3 and SATAIII type. Then your next time will be with the same mobo/ram.
As a related question, can your current motherboard run that CPU now, no matter how poorly? If it can, you could just use that until the 800-series chipset is out. But I guess some of the DDR2 mobos are really cheap anyway.
Strictly speaking I guess that means I'm voting for a variant of the first option.
No, my motherboard can't run any processor that's currently in production. It would be great if I could just pop a $80 CPU but I was unlucky to buy one of the last AM2-only boards so now in order to upgrade the CPU I have to upgrade the MB as well.
Last month I upgraded my other computer, it was a Celeron D on a 945GC chipset and I replaced the CPU with an E5300. Now it's silent, three times as fast and all it took was $70. Too bad I can't do that with this AMD system... I hope AMD doesn't retire AM3 too soon.