I don't trust a 300W stock PSU on a 8800GT card. But hey that's just my opinion, and opinions are just my thought's on it.
As far as the other PSU upgrades, I would just caution to make sure it will 1)fit the stock psu area and 2) have the same pin outs on the mobo connectors. I have tried to replace the stock psu on a few brand name pc's in the past only to find that their mobo pinouts are a little different. They may not be in this case, just a word to the wise, CHECK. If they are the same, hey go for it. That Antec Earthmuffin PSU should be ok or the thermaltake.