Tighter timings will usually give you the better performance as long as the difference in mhz isn't too large. If the crucial wasn't getting that much negative feedback I'd say get the crucial. There are other 4gb configurations you can get that have lower timings for maybe about $15 more.

As for more memory being slower, and on vista, you've got to be kidding me. 4gb on vista x32 will perform better than 4gb on vista x64, even given the loss of not having the whole 4gb available on x32.

Unless the card is too hot or overclocked too far, then you wont have any problems. You can always underclock if you're getting errors/artifacts, overheating or whatever.

And yes, fan speeds are very important; make sure to increase if you overclock or it's too hot in your room.