Please .. put some efforts and brainwork into your question and read through the existing threads in newbies corner. Your question pops up nearly every day .. and the advices are quite consistent. But since you haven't even thought about budget .. :dunno:
I think for your budget 500d would be what most pple recommend. Some say entry lvl some say mod entry lvl, but I think it's fully alright for a newbie to start with the 500d. Not heavy at all, functions are there for basics, ya. Good choice.
Sony A550 is certainly the better camera from the ones you listed.
NX10 is a nice EVF camera, try it 1st to see if you like EVF cameras before deciding.
500D is older and not as good compared to A550 as some functions and performance parameters have become better in the newer cameras (eg. HDR function; high ISO). The comparable model would be the 550D.
specs wise, the Sony A550 is the better one, but it does not have video. So if having video is one of your concern, then u can immediately strike off the A550 and look at the other DSLRS. However if u don mind not having video, well u can go down to any sony style shops and play around with the A550 then decide for yourself. Its also the cheapest, at $799 (Sony usually sell their DSLRs cheaper to attract new users, compared to Canon and Nikon).