Slims right now should not be that hard to find. I know during major releases(like God of War) they are, but you should be fine right now.
Yeah the G25 and the G27 work with the Ps3.
The problem with the G27 right now is that GT5 prologue wont let you use the paddle shifters, and the gear box handle can only go up to 6, so if you have 7 speed cars(like the Ford GT) you can't get into 7th. But there is talk that Logitech and Polyphany are working together to make sure that the G27 works with GT5.
Another great feature about the Ps3 is the fact that it isn't region locked, so you can play games from any region(Which means importing games straight from Japan, if that is your thing). Although this does not apply to PS1 games.
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