I have a choice between a HIS 6870 and a Gigabyte 5870, both around the $150 AUD mark, and both used. (EDIT: The 5870 is $170).
I know that previously, people stated that the 5870 out-performed the 6870, and benchmarks reflected this. It's also apparently a better overclocker. With new drivers and optimisation, is this still the case? I'm not sure which card to go for, considering that I'll be holding onto it for a few years (currently still using a HD 4850 golden sample from a few years back).
look at something in the 78xx range or above in AMD/ATI
you should be able to pickup a 5870 2nd hand for sub $150 ($170 is a bit steep)
A normal 1gb 5870's are going for $100 - 130 and extra good one's (2gb Eyefinity 6, or high voltage models) are going for $130-150
www.overclockers.com.au has a hugh 2nd hand market for all parts and you can often pickup great 2nd hand deals. you need to create an account and wait a little while b4 u can access the for sale section.
I have a hd 6850 which can play any game out there maybe not on max settings but I guess you get what you pay for I do recommend the 6870 because it is about 10 fps better then the 6850 and i can play crysis 2 on ultra at around 20-40 so i guess with the 6870 you would get around 30-50 or more!