which is the best graphics card out of these two ?

I play allot of online games. If you dont mind me asking what is your budget?

Do you have a pci-express x16 slot on your motherboard?
 
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
 
I runa 7600 GT and it was around the 60 quid mark PCi-Express.

Deffo worth paying the extra money if your budget is more than 60 then your onto a winner. I preffer Nvida Cards but thats just me.
 
these are cards ive allready got i have the nvidea and my son has the ati ive had my card well over 2 years now but it can still play all the top games so theres no point in getting rid of it yet

the reason i ask the question is because when we come to play ghost recon advanced warfighter my default video setting for the game are set to low however when my son plays it on his pc everything set to high and he has a lower spec pc than myself how bizarre is that !!
 
You can overclock the 6600 to some quite ridiculous levels, and the default settings are a rubbish guide anyway, just tweak them to what your pc can handle