For one, it seems that it depends on what is in your area. You may want to go online and see what is in the store. Lately external drive prices have been dropping and dropping. Secondly you may want to do that just because only you know what works best for your setup. Like silver? There are silver cased drives, enclosures you can add your own drive to, etc. Some are firewire, some usb 1.0, 2.0, or a combination of both.
For ME Western Digital came bundled with Dantz Retrospect which had some nifty backup software that would let you make an exact copy of your drive and my drive was 80g... First x amount went to mirroring my drive on a weekly basis, 2nd leftover amount (practically auto partitioned using disk utility) is used for a cutting board of sorts.
More than 40g yes, but was cheap enough (under 100 bux) for that 80g. That was just two or three months ago. NOW you can get like a 160 or 200g drive for the same price roughly. So again, look around, but a general question like this is almost pointless. One would need more details to give you a worthwhile answer.
Rey.