6600 GT is going for $65 USD on eaby last time I looked. I hate saying this but with the cost of shipping, even one way, the auction costs and the risk it won't be fixed properly you might only break even re-selling it.
On the other hand the damn thing broke so I would send it in knowing it will cost them more than they made selling two cards to fix it. Teach them not to factory OC video cards. Be aware that if your card is not economical to repair they will send you one off their return/exchange stack that could be ok or might have been abused.
The reason not ot send it back in the original box is that it is not strong enough to survive shipping. You can put the original packing in a stout box and ship it but you will not get the original box back which is a negative when trying to resell the repaired card. Put it in the antistatic bag and wrap it with bubble wrap. Then into the smallest shipping box you can find that it will fit in. You pay to ship the shipping materials too so keep it small and light. The US post offices have small rigid boxes for sale check your Canadian post office. Figure it will take 6-10 weeks to get it back. (4 weeks in transit and 2-6 weeks for repair.)
The 7600 GS or X800Xl are ok cards. The X800XL is faster for DirectX games, if you play open GL games then the 7600 GS is probably faster. I have a X800XL and at about the same price I guess I'd take it over the 7600 GS.
Read my SIG! ATI will not exchange a card you do not like or are disappointed with so be positive before buying from them. Sapphire or Powercolor may go exchanges. Check!