and keep in mind that graph is a lowly clocked gtx 460 at only 675 mhz,
with safety and EASE I would say 99% of 460's can be overclocked to at least 800mhz easily using MSI afterburner.... and then the lead over the 550ti is well worth it
On my old gtx 460 768mb (which I now gave to a friend) I had it at 920mhz easily with a 100mV voltage bump, almost no added temperature and the noise was still in-audible. That gave me a 30% increase, to about ati 6870 levels
i would take the 5770 over the 550ti for obvious reasons, it uses less power but gives the same performance. but if you can find a 460 that fits, go that route. Keep in mind the 550 and 460 use a decent amount of power. a Good quality antec/seasonic/corsair 450w PSU should be used. Dont use a no-name brand. The 5770 uses the least powert and can be run from a strong 350w unit.
^ the power "requirements" you speak of are "recommended" not requirements. They put their recommended psu higher as many cheap PSU manufacturers overrate their psu's. An example is a Diablotek psu rated at "600w" cant even produce 300w in real world testing, where an Antec 380w can produce a little above its rated power. Nvidia and AMD have to overrate their recommended PSU wattage to make sure their cards will run even if you have a crappy psu.