Tbh I would prefer the e8400 over the entire Phenom lineup. It would depend what you are doing. Well optimized programs for quad core will probably point to anything 2.4ghz and above on phenom x4. However most games and other programs will point to the E8400 (especially when oced).
If you are talking about phenom II however that is a different story.
The phenom II X2 550 BE would be comparable, though I believe the 8400 would come out a little ahead more times than not. The problem with comparing it to a quad or tri core is that in apps optimized for multiple cores, the 8400 would be blown away (at stock speeds anyway), but in games and apps that don't make use of more than two cores the E8400 would beat out lower clocked quad cores.