Get a 6600gt. You will need it for SM3. If you cant afford that, get a 6600, and if not that, get a 6200, but please do not get hypermemory or turbocache. WHat it means is that any video card that has hypermemory or turbocache does not have any memory on it, it instead takes system memory and uses it for video memory. I guess if you have like 2gb of RAM it would not be a problem, but system memory is much slower than video memory.
BTW, having 256mb over 128mb does not cut down power needs. Don't get an ATI card, they do not support Shader Model 3 which is being used in a lot of games that are coming out and I know there is a mod coming out for half life 2 that will use it. It might not make a difference to you now, but it will hit you hard later.