Short answer... None. There really isn't a
good wireless surround system. Interference is always an issue, and tends to degrade sound quality.
In your price range you're looking for a HTiB (home theater in a box), from wht I understand some of the better ones are Panasonic, you're probably gonna want to start there. Panasonic has a HTiB Blu-ray system (
SC-BT200 ) for $599. If you don't need/want Blu-ray then DVD model can be had for less, just head over to Panasonic's website they have a lot of options, with or without towers, wireless rear speakers, etc.
Now if you're willing to save up some more then you can get some of the
real good stuff
.
ETA: The "wireless" speakers typically still have to be connected to a wireless receiver box (which is powered), so it's not really a true wireless, but at least you won't have to run wires through the attic. And it is almost always just the rear speakers that are "wireless".