Hard drive question

Ok so I know that if I take a regular external I'd been using on windows and plug it in through USB, it works fine with my new MBP...what about an internal? I see "mac drives" seperate from regular say, Western Digital laptop drives on newegg.com and zipzoomfly.com, if I buy a 500gb WD laptop drive to add to my 320gb, will OSX format it for me and have it working fine? Or do I have to buy a mac-specific laptop drive?

There's no such thing as a Mac-specific hard drive, Macs use the same type of notebook hard drives as other manufacturers. As long as you buy one that has the correct specs, then it'll work fine, no problem. I upgraded my internal hard drive with a bigger and faster one and it worked out great.

Specs like what, just making sure it's a 2.5" laptop internal? Or power usage and random stuff like that?

Just that it's a 2.5" SATA internal laptop hard drive with a height no greater than 9.5mm. Cache and rotational speeds are discretionary (I chose to replace my 5400RPM drive with a 7200RPM drive).