I'll assume US not Canadian, Australian...
Also, what resolution is the monitor?
Right now the HD5770 would be a good choice, anything faster would need a more powerful PSU and be held back by the CPU.
If you plan further upgrades ( CPU/RAM/MOBO) within a year, or you play at 1920x1080 the other suggestion would be a HD5850+PSU. Whith a little careful shopping you will get both for less than $400 US and they can be carried over into the next upgrade, but that card will be wasted money if you game at 1280x1024, in which case the HD5770 will be plenty.