The best deals I've ever found have been from specials in Sunday ads at local computer stores, and anything you buy online will probably be significantly more expensive. Given the nature of this, what you get varies from week to week, but the best ones in your price range from my current weekly ads are: Core i3-380M + 4GB for $530
Pentium P6200 + 4GB for $450
Turion P560 + 4GB for $500
All of these are about $100-150 below the list price, so in this price range you can get a lot more from a weekly special than you would from buying it online or direct from the manufacturer.
You probably won't have much choice in brand, but from my experience: HP and Sony have made good laptops that don't break.
Gateway's customer support hardly ever does anything except tell you to wipe and reinstall (even if the issue is obviously a hardware problem), and the one computer I've had of theirs was literally falling apart after six months of moderate use (and virtually no abuse). I don't recommend Gateway at all, although YMMV.
None of the batteries in any of the laptops we've had (HP, Sony, Gateway) have worked that great. One of the Sonys doesn't hold a charge at all after 2 years, and the other holds about 15 minutes after one year. The Gateway never held more than an hour (if we were lucky) and now holds about 40 minutes after a year and a half. The HP held a couple hours for several years before it fell into disuse, and now holds literally no charge (though it's about 8 years old).