You'll pay more for the same level of performance.
Nvidia's main advantages lie with CUDA which helps give it awesome (as in 2x) F@H performance when compared to ATI, Phys-X, which is available on a few titles but hasn't really taken off yet because it's an Nvidia exclusive and most game developers aren't going to use the resources to support something that isn't an industry standard, and drivers that are arguably easier to use.
The only reason that I chose Nvdia for my current build was because I found two 9600 GSO 768MB cards for $40.00 apiece and a 780i SLI motherboard for $116.00. Phys-X, F@H performance and a driver interface that I'm used to are just nice bonuses.
For around $100.00, ATI rules the roost when it comes to getting the most bang for the buck. If you like Nvidia, though, look around for an 8800 GTS 512MB, 9800GTX 512MB, 98000GTX+ 512MB or a GTS 250 512MB. They're all pretty much the same card and in he same performance category as the 4770/4850. For slightly less performance you could try an 8800GT or 9800GT. Whether or not they will run on that 400W PSU of yours, I don't know, but I believe that they draw a little less power than the 4850. I still think that the 4770 would be your best bet, though, because I know that it would
run on it. Its performance per Watt is probably the best of any video card on the market right now.