Laptops in that price range that can game and have good battery life... well firstly you'll have to say what screen size you're looking for then I could make some suggestions on what'd be decent and also say whether you want to buy new retail or consider using eBay etc (can be iffy). As Heart said about battery life if you're plugged in most of the time it'll kill out your battery as they don't like to hold their maximum charge for long. Sony is one company that got past this by having Batterycare which enables you to set your max battery capacity at 50% if it's plugged in the whole time to preserve and lengthen battery life and then you can turn it off and have your full battery capacity when you go on the road. This said it's rare and I've heard of something similar in a couple Sager's (Clevo rebrands). Aside from that you can of course plug out your laptop battery and store it at 50% charge in a decent temperature bla bla place but of course that then means you should be a little careful with your power cable
Sony are expensive and aren't an option for your price range really (well at least to get something worth while). So yea let us know what you want from a laptop or go the route Heart suggested and go netbook + desktop! I'm in the works to sort my own build and also looking into possibly buying an Asus 1215n as it's a compromise between not having awful performance, decent battery life and experience but also price at around £430. To let you know I've used laptops all my life and have been researching them extensively for a while and now that I'm getting money together a desktop won out logically, so I'm not just spouting crap at you. Good luck.