I have the 4770, you can o/c it about 20%. It has the 40nm new smaller die the 5 series cards are using. It never gets hot. I have the xfx one available at newegg. In RE5 benchmark at 1680x resolution high settings, 2xaf I get 68fps, with the oc I get 85. When I bought this, I was going to get what I could get the cheapest.
In most situations and applications, the GTS 250 will win. So in a budget gamer or something of that sort, that would be the way to go.
The 4770 does have the advantage of using less power and making less heat, so in a media center type application, it would fair better. Also, this has already been pointed out but if you are into OCing, the 4770 would be more happy to oblige than the 250GTS.
I do want to point out though that if your PSU has 2x6 pin connectors and you can spend another 20-30 bucks, you may want to look at the 4870.