[xubuntu] Most games will not run

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http://ubuntuforums.org – I'm aware that this is a pretty vague description, but it's the best I can do: When I try to open games through the applications menu, nothing happens. I've used top to check if any processes start running, but they don't. I've been trying Braid, Bastion and Nexuiz, and all have the same problem. Games like Aisleriot and Mines run just fine. I believe it might have something to do with my graphics driver. I'm using an Acer Aspire 4820TG, which has a Radeon HD 5650. I've installed the proprietary driver through Additional Drivers, but cannot install the post-release updates - Additional Driv (Hardware)