I would get the best ram you can afford, as in pc3500 or something, that way you will have it for future upgrades and wont have to go out and buy new ram. That is just what I would do, so if you are limited by budget, or the system you are on, get the cheapest stuff I guess.
the rated speed of ram only means what its SUPPOSED to run at, getting PC2700 and running it at 133 FSB is useless. unless you plan to OC as much as you can, use the PC2100. in any case youll probabily get a lot more performance gain out of the 1 gig at slower speeds than the 2700 at slightly higher speed