256 is fine for just about every game up till now for 1440x960, even with AA on. But for future games, and to some extent crysis, 256 will most likely become insufficiant. I would try and get at least a 3870, or wait for the Powercolor 3850 with 512mb.
It depends on the make and 12v rail amperage to determine if the PSU will be enough. It might be, but you might be pushing it though.
1) It should be, as a general rule of thumb is that anything up to 1280*1024 doesn't really need more than 256Mb of vRAM. Having said that, quite a few new games (Crysis etc) would benefit from 512Mb. I haven't actually seen any benchmark comparisons on the difference between 256Mb and 512Mb on Crysis at 1280x1024. Anyone know of any?
2) It should be ok but the main thing is the amps that are supplied on the +12V rail. Check the label on the side of the PSU/PSU box.
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.