Well, I have a MSI one and OMFG!!!111one!!
Few reasons I bought it:
1. MSI. great company
2. 6600gts run HOT and this one has one massive, copper HSF
3. 2/3 year warrenty instead of 1 (though lots of companys have more then 1 year.)
Bonus: comes with Prince of Persia: SoT!! (and XIII, Uru(think thats what its called), and plenty of other games/apps
The only other company I would recommend for the 6600gt is BFG. Lifetime warrenty, OCed out of box, and all in all BFG is a good manufacturer. But Newegg doesn't have the BFG model and they are around $50 more when you find them.
Go MSI. You won't be disappointed