Seriously, some AM2 boards can take AM3 chips. I find, cpu-upgrade.com handy.
If Phenom II x4 works, this system can hold out until Bulldozer.
Unfortunately, it does not.
If you are inclined towards the AMD side, I would say a $150 now would be good and if you can last till Bulldozer which is rumored to be delayed till second half of 2011. But since SB is coming out earlier i would say wait till then and get one of those.
Is there a particular CPU/Mobo combination you would suggest?