The HD 5770 is newer, faster, supports DX 11 and eyefinity, and has lower power consumption than GTS 250, so it is the better overall card.
For the ram, you CANNOT have the choice between DDR2 and DDR3, your motherboard will ONLY support one of them, for example if your cpu is core i7/i5/i3 then you MUST get DDR3 ans DDR2 won't fit on the motherboard.
Gaming performance is give and take.
I would at least give the GTS 250 a chance if you want to save a little money
I've read good and bad about the 5770 and the only thing that I don't like about it is the 128-bit interface.
I owned the older 128-bit interface cards and even the touted 7600GT was by no means able to overpower a 256-bit card.
Excuse me if I seem undereducated here but I'm just speaking of past experience since I've owned what at one time was the 'latest technology' just to be overshadowed by todays offerings.
You have to understand I live in the half pocket.
Somewhere between the ground and the air and just floating on what I know and read. It's getting better little by little so bear with me.
Look up benchmarks for the cards and youll find that most cards will be older but better... DX11 sounds great but atm i think only the new stalker uses it, so its not a real bonus at this point in time for gaming, only for any 3D animation and rendering.