+1 to jjb on the GTX670 actually - that's what I'd do. I have a Radeon so haven't had a chance to enjoy PhysX first hand, but check this out:http://www.youtube.com/watch?v=kAfzjTAhpBk&feature=player_embedded
The AMD fans always go on about PhysX making no noticeable difference and being a gimmick and I took their word for it. Figured the really strong arguments for nVidia were adaptive v-sync and more consistent delivery of framerates (as opposed to inconsistent up/down performance). But I think PhysX actually looks like a genuine selling point, kinda jealous now.
Brand-wise, I'd grab an EVGA, but they don't do Radeons. If you really want to stick with the Radeon, I'd recommend the Taiwan big three - MSI, Gigabyte and Asus (in that order). I'd steer clear of Sapphire (I've had too many bad experiences with them) and Powercolor (same). VTX3D are the same guys as Powercolor (Tul Corporation). I have an XFX currently - it's fine except the fan is faulty. HIS I've had no experience with, but I've seen quite a few threads on here about problems with them.