Ill give some suggestions in just a second, but in terms of where to buy, I would shop on Newegg and look at the reviews, as most people there have good technical knowledge and you can pick out good details from their reports. Also, make sure to go to the "homebuilt" area and check the forum about "Read this first", it has a good section about hard drives I think and explains what you need to look for.
Here is a quick list:
RPMs (Rotations per minute), normal notebook drives go at 5400rpms, but desktops and newer laptop drives run at 7200rpms. Obvioiusly, the faster the disk is spinning, the more information it can read in the same time (more area passing the reading head per second), so the faster the drive is.
--Another important point is disk density, but this info isnt listed normally, youd have to look up the specifc model number and see if the manufacturer lists it (WD, Seagate, etc). If there is more data per in^2, then more data passes the reading head per second, so it goes faster. This info can sometimes be found in someones review of the drive
Cache. This is a small amount of memory that stores the most recently used files and can pull them from the drive REALLY quick. With numbers-Im-making up now, if it takes 1second to pull data from the drive, but the file has a copy in the cache, the drive will pull it from the cache and it takes only 0.1 seconds (all made up numbers, but cache is WAY faster). Only a small amount can be stored in the cache, so the more cache space, the more files can be there, the faster your experience is. 8mb on a notebook is standard, but with larger drives (500GB+) Id look for 16mb. Desktop drives (my 2tb drive) has 64mb.
Energy consumption (usually you have to look this up on the manufacturers website when you narrow it down to a few choices). Two almost exact drives could use wayyy different energy. This will kill your battery faster. 7200rpm drives usually use more energy (spinning faster)
Reliability. Look at reviews. If it has pretty good reviews, buy the drive. All fail, what you want to do is just test the hell out of it before putting data on it (write it all to zeros (which makes sure to try to write every single sector on the drive) and check the SMART values (look up how to do that on google). It will tell you off the bat if the drive is bad. Message me if you need more detail on this
My suggestions: I purchased the WD Caviar black 500gb and like it. Its 7200rpms (fast) and has 16mb cache. Good reviews and also uses less energy than the seagate one that is similar. Strangely, this doesn't use much more energy than a 5400rpm drive. Ive been using it as the only drive in my notebook and its great.
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136692&cm_re=wd_500gb_notebook-_-22-136-692-_-Product
The seagate drive I mentioned above is alright, but the wd was better imho since it used less energy and still had just as good of performance, but just to list it in case:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136692&cm_re=wd_500gb_notebook-_-22-136-692-_-Product
This seagate is a hybrid drive, meaning it uses Solid State for cache (meaning any of those files in cache that are used frequently and come out fast come out WAAYYY FAST, so programs literally just SNAP open if you use this as your primary drive and have programs on it. With just a data drive this may not be worth the money, but up to you)
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148591&cm_re=seagate_hybrid-_-22-148-591-_-Product
Hope that helped a little!
PS: Solid state drives are screaming fast, so if you use an SSD as your primary for programs on OS then any program and the OS would run unbelievable quickly, but its REALLY expensive for not a lot of space ($100 for 60GB on a deal).
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