Pc2-5300 667 v PC2-6400 666

Yes, as long as it's physically and electrically compatible (number of pins and voltage), it should work, just not any faster than the system is capable. It won't hurt to try it. If you have problems with booting, a flashing latch light indicates a RAM issue:

http://support.apple.com/kb/HT2341

PC2-6400S-666 is 800 MHz RAM. 6400 means 6400 Mb/sec bandwidth / 8 bits per byte = 800 MHz. The '666' refers to the latency. So, you have bought DDR2 800 MHz RAM for a machine that can use only DDR2 667 MHz RAM. I believe you got RAM for an iMac, not a MacBook Pro. It might work (though it will only run at 667 MHz), but I'd wonder what else might not match - you'd need to confirm that the RAM you bought is 200-pin, not 240-pin, the correct voltage, etc.

You are probably better off just buying the correct RAM for your MBP - try crucial.com's memory selection tool (even if you don't buy from them).

It should work, but it may not. RAM is picky sometimes. I don't know why you bought the faster RAM as your FSB is only 667MHz anyway.