Well if the tower is a slim desktop (SFF or Slimline) then go with the HD 4650 since it is a low profile card
Personally I picked the HD 4650 a while back when faced with the same problem
(own many Dells)
low power card but still can handle some gaming if the need ever comes up
a great multimedia card for a Home Theater PC
but also the HD 5450 would work good for your needs and consumer less power
but not as powerful as the HD 4650
another one to consider would be a HD 6450 which is the newest technology
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127610&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-RSSDailyDeals-_-na-_-na&AID=10521304&PID=4176827&SID=pw4xdh7u29euthat is the low profile card
most low profile cards come with full size and half size brackets
so can be used in full or half size towers
the HD 6450 will have the latest feature set as far as UVD which is used for hardware
video decoding which uses the card for accelerating your video playback