If you go for a GTX 460 get a model with at least 1GB of memory and maybe a factory overclocked one?
If you want to future-proof for 1.5 years it is a tough call. I am going to assume that you don't have SLI capability on your motherboard - what about Crossfire with the 6870, so in say 1 years time you can always bung in another 6870 if you felt the need.