Basically its a waste. Here's some fps charts using various RAM and timings:
http://www.tomshardware.com/forum/200494-31-tomshardware
Notice how there is little to no difference between them. I would suggest to overclock your RAM and loosen the timing but I doubt a Dell laptop has overclocking capabilities.
The only real reason to get faster RAM is if you overclock your processor with it:
jaguarsk wrote...
If you don't overclock then the DDR2 677 and 800 will more or less provide you with the same performance. You'll need to benchmark if you actually want to see any difference.
On the other hand, overclocking in a different story. Faster RAM will allow you to overclock more. For example, the Core 2 Duo E6600 runs at 2.4GHz using DDR2 533 RAM. That's 266MHz x 9 clock multiplier.
DDR2 667 will allow your E6600 to run at 3.0GHz (333MHz x 9 clock multiplier).
DDR2 800 will allow your E6600 to run at 3.6GHz (400MHz x 9 clock multiplier).
Also laptops generally run hot as hell and overclocking will only make it hotter.