Out of the cards you listed, the 4850 is the best. Now that you aren't even talking about that card anymore, I vote the the gts250. My EVGA gts250 is an extreme overclocker as well. I can overclock to 825 core, over 2k shaders, and 1350 memory.
The gts250 is slightly better than a 4850. 4870 is better, but not really that much better.
As far as crysis goes, you really need a quad core over 3ghz and atleast a gtx260 to run crysis ''smoothly''. You need faster memory and more of it. Look into getting a 2x2gb dual channel kit of DDR2-1066 ram with 5-5-5-15 timings. That will be a worthwhile upgrade for you. Thats if your board supports it, some older 775 boards will only support ddr2-800 ram. If thats the case, look for 2x2gb of 800 ram with 4-4-4-12 timings.
But, with a 4770, and the rest of your setup you should be able to run crysis on medium settings.