So my Macbook Pro is coming and it's a 15in Core Duo 2.16Ghz with 2 cores. It's already up to 2GB of Ram which seems to be the max. But the HDD is only 100GB. Is there a certain limitation I can go up or can I go has high as I want? Because I was hoping to get a 250GB which is the perfect amount. The reason I ask this is because I heard from someone that there is a limitation.
From what I could gather from OWC you are right about the RAM. As far as hard drive, you can upgrade to whatever capacity you want as long as you get a 2.5 inch drive (sometimes labeled as laptop drive). There are guides ifixit that will show you how to do this. 250 gb is nice because they are so cheap these days.
Okay. Next and final question. What would be better. a 250GB 7200RPM HDD or a 500GB 5400RPM HDD? They're both the same price or very close.
As with everything, it depends. If you need maximum speed you should go with the 7200 RPM drive, if you need a lot of storage for stuff like video, raw images, music then go with the larger drive. The difference between 7200 and 5400 is not that big.