Well, since you already bought the HD5770 then stay with that card, why you want to buy another card (GTS250)??
What is your monitor?
What speed is your CPU? you can OC it a little to get more performance increase...
My Graphic card now is 8600 GTS 256 MB , i bought the HD 5770 Vapor-X GDRR5 and make bottleneck in my system , people said me that my computer is old for this card and with OC maybe can work good . i thought that maybe the GTS 250 could work well on my computer. My monitor is a CRT 17 " Im player of counter strike and left 4 dead and i play with low resolutions. And the subject in OC the problem is that I am a newbie and I have fear of doing it wrong. Sorry for my english .