400w in itself may be enough, but the amperages on the 12v rails might not be... generally you want around ~25amps (or more) on the +12v rails (definetly more if you have the option to) (also look for name brand psus if you can, the psu forum is a good place to ask to find out which to look for)
as far as memory, as long as the brand is reliable, then by all means (brands such as corsair, patriot, OCZ, mushkin come to mind)... 2*265MB is more of in inbetween upgrade TBH... but itll definetly offer some improvement above your current 1GB
avoid off brands if you can, because you have no way of knowing if it has been really stress tested for problems, which can end up causing all sorts of weird issues then