the newer time capsules have the following features you won't get by replacing the drive in your old unit
1) the newest units have 2.8 x the maximum power output over the previous model (392mw vs 192mw)
2) the newer time capsules have certified/official/final 802.11n code the older ones were draft n 2.0
3) the last two versions of the time capsules have 3 x 3 spatial streams, meaning you can do 450 mbps over 5ghz
4) the newer time capsules might have simultaneous dual band, your old time capsule might be one of the older ones that made you pick what frequency to use
the newest "md031" time capsule has a maximum power output of 392mw versus 192 mw for the older model
that might give you better range or a faster speed at the same distance as the older models. i told the internet this and those ifixit mac insider guys finally realized what a blunder they made reporting "no new features" and promptly ripped me off and didnt credit me for my airport discovery!
the time capsules might of been always dual band but only the last two were simultaneous dual band.
with the oldest ones you had to choose if you wanted 2.4 ghz or 5 ghz but you couldn't choose both of them at the same time, the recent time capsules have 2.4 and 5 ghz at the same time
i'd say if you have an older time capsule find out what model it was and if it was older tehn the md340 then sell it on amazon for $100 to $150 and use the money to buy the newest model
and then thre was that whole mess back in 2008 or 2009 where all the time capsules were only lasting 18 months because of a probblem with its power supply?
if you already have a "md340" time capsule and if you wanted the latest "md031" model then i would reccomend buying the newer model from some place with a good return policy like amazon and testing it in your house and to see if you think the improvements is worth the $299, and if it is you can keep the newer model
and then sell the older model