You've little control of the actual temperature using a toaster so use an oven.
As Vrumor points out, without proper commercial reflowing equipment it's a temporary fix at best and quite a few home ovens can't get hot enough to melt the lead free solders used these days anyway.
If the card really NEEDS to be reflowed it's probably well past its use by date, either look for your original invoice and see if you can RMA it or start researching a replacement.