hi,
i installed a new 7200 drive in my blackbook a few weeks ago. im finding that there are fewer little hangs, and i haven't had an overload message in logic since, although admittedly i dont push it to the limit very often. i had already installed 4g of ram so cant tell you about any differences there - ive never run logic in this machine with less.
this is the drive i bought:
http://www.misco.co.uk/applications/SearchTools/item-details.asp?EdpNo=320046&CatId=0
(i only paid for the cheapest delivery option and it came the next day - if the item is in stock and you order in the morning i expect anyone else would have the same experience)
i believe that the bigger system drive, the better! im sure that the more empty space you have, the smoother your system will run. it seems to be the case here, at least. im not experiencing any problems with heat, in fact i dont feel any difference whatsoever. the drive is as quiet as the stock fujitsu drive that came in the laptop. Battery life (iv just checked it) is still very good and the meter is currently showing 4:44 on a full charge, which is probably less than before but good enough. i now have my old drive in a el cheapo case that i ordered at the same time, made by Sweez. its working fine in that enclosure.
my process was to fit the 7200 drive in the case and plug in using both usb ports (the appropriate cable came with the case) then create a single GUID partition on it using Disk Utility, then clone my system drive to this using CarbonCopyCloner. This took about 3hrs with 120Gb to clone. Next i checked the new drive and its system (without removing it from the enclosure) by selecting it as the new Startup Disk in system preferences, and restarted. No problems. Finally i swapped them over, a procedure which was easy and took about 10min. Laptop has been working fine ever since, and im using the old drive as an external. Im keeping my original system on there for backup, and iv added a slim version of my BFD2 samples, which is working out surprisingly well.
So i have a faster computer with more space than i need, and a new (old) external for backups etc, and it only cost me £75. Pretty good, i think. I hope that description helps you decide whats best for you.