we need your computer specs
when picking out a graphic card you have to know how big your power supply is
faster cards need bigger psu's
if you dont have any 6 pin powwer connectors i would suggest 7750
At that Resolution I really wouldn't recommend anything more expensive than a 7750 or 5770...7750 doesn't need any extra power connectors, just as alvine said...but the 5770 does give you a bit more performance for about the same price, but, it requires 1 six pin...
I wouldn't even look at anything more expensive then those options It would be a waste of cash at 720p.
But if You want to be solid for games releasing 2 years down the road, and your PC can handle it, then I would recommend something like a AMD 6850/70 or the 7770...but that's if you really want it to last 2 + yrs down the road while playing on max/ultra settings
the 7770 is just not a good deal if you can get a 6870 or gtx 560 for liek 10 dollars more... not worth it unless you are running under strict power requirements which is sounds you get the right idea of getting a solid 5-600w unit.
with the cards that I rec. above at least you will get playable frame rates if/when you decide to upgrade your monitor to 1080p with something like a 6770 or 5770 it will still game at 1080 but not nearly as good as the gtx 560/6870
..and since the 7k series just got released, you can get a good deal on a previous gen card that is more powerful for the money in that segment.
Now if you wanted to move up to a 7850 then id say go with the 7 series but for your budget and performance needs Id say that the 6870 or gtx 560 would suit your needs quite well
No it's not overkill. It's a great CPU that will last you 4+ years. About waiting for GTX 650: it will take at least a month for it to get released, but if you got the time, sure, you can do it. Also, if you really want to play games like BF3/Crysis 2/Witcher 2 on ultra, don't get anything less than GTX 560. I understand that other people think that it's overkill, but it really depends on which games you're playing. For something like Deus Ex 3 or Mass Effect 3, it is overkill, but not for the most demanding games