Here I'm trying to discuss about optimal performance but absolute value for money
Brand: Must be top brand offering warranty at least for 2-3 years, don't confuse Top brand is totally different from popular brand(cheaper its popular will be in India most here don't mind performance). These days manufacturers are offering 5 year warranty too.
Connection: Can be two ways Firewire or USB. These days USB is sported high cause can be connected with most computers. But choose what best suits you cause Firewire's performance is unmatchable to USB.
Speed: Seek time <10ms | Buffer Size > 4Mb | RPM : 5,400 - 7,200
(Please keep in mind values here are for optimum performance, go higher if you can spend)
Size: Must be greater than 80GB, as said before if you can spend go for bigger but not biggest it will degrade performance.
Search: If you reading this post its obvious that you are searching sites for idea, but don't stop here. Go to Manufactures site itself compare with their own products. Wide information is available these days use them.
How / Where to get: In Chennai, Ritchie Street (close to 'The Hindu' office) is best to buy but only if you have a expert by your side else opt from showrooms like Croma. Every manufactures are having their own retail shops to check them too. If you are buying in website make sure you choose 'Cash on Delivery' for payment unless you know its a trusted site ;)
Happy in meeting you with another query, many friends of mine owns external hard disk drives but what most of them does is analyses and discuss about it after buying, that's not a great habit to have. But outta those discussions I have brought some ideas, hope they helped you.