Like the others, I suggest looking at 1280x1024 as a good benchmark for what you'd expect for performance. For some reason game sites ignore the relatively popular res of 1440x900 due to the popularity of very inexpensive 19" WS LCDs, but, whatever. Mathematically speaking 1440x900 is quite a few less pixels and in some games that makes all the difference as to whether or not you "need" over 256mb.
A year or more ago, I'd say you were fine, now we see games using that 512mb deeper textures and effects that don't have as much to do with resolution.
Such as it is, I wouldn't say 512mb is a "waste" for that reason.
As for power, I'd say 460 is probably enough for the 3850s, but I prefer to add some insurance to my system by going over the generic brand PSUs especially when you can get a pretty good 500w cooler master for like $40-$50. Your PSU is probably one 12v rail, which if you're overclocking is gonna be pretty strained with today's cpus. So it also as much matters what your CPU and everything else is.
Given the price differences, and assuming your on a budget get a 3850 at 512 and put the savings towards a nicer psu.