No matter what card you choose you'll be just fine. I have a Gts 250 from xfx and its great. I play source games too. From what i herd these 2 cards vary in very small ways. over all gts 250 comes out on top.
My card can handle any of the source games you just listed with unnecessary anti aliasing going at qx16 or what ever it is called ( ha ha ). My fps i normally get ranged from 85-120. ( 120 is my lock so im not to sure )
I think of it as long as its above 55 you can't tell a difference from 62 from 55. Its still silky smooth gaming. Games i play with my card...
Resident evil 5 high detail
Alien vs predator 2010, high shadows/med textures. Dx9
Any source game really.
I think ill go with the 5750.The 5770 seems a little too expensive. Unless you guys think I should just splurge and go with the 5770. I just want to get the " Best Bang For my Buck " Right now I only 25 FPS in Gmod. So, the 5750 is better then a 4850? ** Edit ** I just looked at the specs of the 5750 and 5770's, the memory interface is only 128 bit. That confuses me.
The HD5000 cards use DDR5 ram. They actually have higher memory bandwidth than either of the HD4850 or GTS 250.
The HD5770 is worth the extra money if you can afford it. It is the current generation equivalent of the HD4870 or GTX 260 rather than the HD4850 or GTS 250.
Besides Source games are not that hard on the systems requirements- My dear departed 256Mb 7800GTX could play all the HL titles at 1920x1200 and still stay over 30FPS (although it COULD struggle a little with AA )
Besides, the HD5770 has a longer future, just do n't expect it to play Metro 2033 at very high, DX11 and 1920x1200!