If you are using your old AM2/Am2+ motherboard keep in mind you can use a AM3 cpu but it will draw more power than it would in an AM3/AM3+ motherboard due to they way the power planes and pin control for power states are setup.
I didn't know that - thanks. Is it objectionable purely as a heat/noise issue, or is there some stability or other risk to the system by using a 125W processor in an AM2+ board?
It is definitely a heat/noise/electricity cost issue. Depending on the MB it can also be a stability issue, compatibility issue (the BIOS won't let the system POST with a too high TDP part or might misidentify the part and run it at the wrong multiplier and FSB speeds), and in the worst case a fire hazard if somehow a poorly designed board overheats the capacitors and such.
I used a 95W X3 720 BE (an AM3 cpu) in a AM2/2+ board for quite some time. Just be sure to check the MB support website and/or marketing materials to see if the board was designed to handle processors over 95W, some low to mid range boards weren't and some mid to higher end boards were designed to handle 125W. The ones that were designed for 140W are even less common.
As to the $150 comment I went with the ASUS M5A97 which is more of a $80-$90 board when I upgraded the last time. I'm thinking I'll grab a FX-6300 when they drop in price some time down the road. Until then I'm rocking my Heka though you might want to find a Deneb if there isn't much price difference. Either way I'd be comparing prices on the 65w and 95w parts with 3 and 4 cores and avoid all the 125w and 140w parts.