hey mate, i feel your pain. our prices for computer hardware sucks compared to the rest of the world so yeah....
anyway id suggest you get the 6870 and if it doesn't live up to your expectations then get another one plus a new mobo and crossfire it
If any of your games use PhysX, get a GTX460. If they don't, or you don't care about it, get a HD6850. Use the difference in price between that and the HD6870 to get a quad core CPU. You'll want that if you expect to get the most benefit from possible future Crossfire. Your selection looks pretty limited though; I'd choose the 955 over the 640.
I don't know which version of the GTX 460 you're talking about, but if its one of the custom design cards from ASUS, MSI or GIGABYTE, then I'd definitely go for them.They allow for better overclocking and when overclocked to their fullest, they can match the HD 5870 in performance.Otherwise go for the HD 6850.If you want Crossfire in the future, you might have to go for a quad core.