@Loxli: The quickest win would be to open your game to closed testing and provide an easy way for people to post PC specs and/or bugs while they are testing. Based on that information you could then quickly be able to optomise your game and include the community at the same time.
Having said that however, if you have the cash to burn then Erasio’s suggestion would work. I personally prefer to use Guinea Pigs … errrr … testers … as this gives me access to a wider range of hardware and allows me to simulate real-world rigs without having to buy any. 8-}