Hard choice. Both the 4850 and GTS250 are solid cards that are still a good bargain today. 4770 offers near 4850 performance but at 30W less, which means cooler running too. What are the approximate costs for each?
Amazon can have some great prices, and they are good. But, I've been shopping at Newegg since 2004ish and I've spent quite a large amount of money there. I've NEVER had a problem with them. Fast shipping, good prices. If you're in the US and arn't going to pick up the items your self at Microcenter or Fry's, I would recommend Newegg over anything. Amazon can get some crazy good deals, however.
But with Newegg. the GTS250 is cheaper (and its a great version).
If Newegg is cheaper (and you are in the US of course) go with them for sure. They have been very good to me in general (bending the rules now and then to make things right, extremely efficient) with only one or two messups (poor packaging, but it appears to have been remedied).