Well, about driver problems with AMD cards, usually it's just with crossfire/dual-gpu systems, for single-gpu systems there's not that many problems.
I have an HD4850 on my PC, my brother has one as well on his, my parents PC has a HD4670 and my notebook has an old Mobility X1600, neither of them ever had any major issue with drivers.
Anyway, the Nvidia cards:
GTS450 is comparable with the HD5750 in performance, but has higher power consumption.
GTX550 Ti has a little bit higher performance than the HD5770, but higher price and higher power consumption as well.
GTX460 1GB version (not the GTX460 SE) is comparable with the HD6850, but has higher power consumption and it's more expensive.
About cards with lower power consumption so that you don't have to change power supply, there's the new Radeon HD6670. If you're not in a hurry to get a new card, I suggest waiting for it to lower a bit in price.
One question though, what is your current card?