The 6600GT is the better of the two by a fair margin, ignore the 128mb difference on that spec of card it wont make a difference
Can i ask how much you're paying for the card, as its not worth more tha nabotu 35 quid mate
as for a pcie 16X slot, if you're looking for a cheapish hard hitting card id try and get a 7600GT, real good card and if your lucky u can have one for about 60 or so quid