to answer your questions:
they are woofers only - 5.25" and the cross-over to send them only low freq is built into the amp. In my '99, I had 2ohm speakers in the door but 4 ohm in the dash (I think some later MY might have 4 ohm all around).
Factory amp (in M490 option package) is only 40W per channel so you can put "big watt" speakers in if you like, but you need to make sure the RMS low end rating is in the neighborhood 2-4W and the top line is at least 40W. Otherwise, you won't give them enough power to make them sound good/right.
Another important aspect is the freq rating of replacement speakers - you want something rated 50Hz or below. I don't know the cross-over point but anything higher is basically mid-range and the speakers won't get the signal they need. The crux of the problem is finding 5.25" speakers that meet the RMS and freq requirements that can be plug n play w/ the factory tuned port enclosure. 5.25" - especially at lower power ratings, really don't go below 55Hz or so. When changing out mine, I tried 3 different sets of good quality 5.25" speakers to swap in and 2 sounded worse than stock, the other was only as good as. So I ended up pulling out the tuned port and putting in 6.75" Infinity Kappas.
This is much easier than it sounds, but what you might want to do is build custom baffles and mount either 6.5" or 6.75" two ways (only wiring up the woofer, though) to replace the tuned port enclosure and the stock 5.25" speakers. I did on mine and the sound is exceptionally good - but it all looks stock and uses the stock amp.
Here are 2 links:
How to Modify the Boxster's Sound System by Andy M on RennTech
Template for Custom Baffles
you don't have to use the template I created in the second link but it might help. Otherwise, Andy's directions are spot on and each of the projects (dash, rears, doors) will take an evening (about 3 hrs) to complete - start to finish.
Good luck and make sure if you are going to keep the stock amp you find speakers that will maximize it's power - RMS, peak, sensitivity (91dB +) and freq. Take a look at Crutchfield for more information on ratings, etc. if you're not familiar. You can find prices better than them but they do have some good educational articles to help you understand speakers better.
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