I'll try to help, but as nRollo just said, this question is asked several times a day here.
I happen to have an 8800GTS 512 and a GTX 280 1gb here in my two pcs.
If you are not concerned with AA, the 8800GTS 512 will play pretty much every game available now at 1920x1200. (with the exception being Crysis of course)
Going to a GTX260/4870 1gb will just allow you more "headroom" so you can enable AA @ 1920x1200.
Me personally , after installing the GTX280 and playing Fallout 3, Far Cry 2, Dead Space and Call of Duty World at War, I told my wife that im not so sure i did the right thing because my other pc with the 512mb 8800GTS plays all those games just fine also.
So for
CURRENT games the 8800GTS 512 is plenty fast enough if you dont use AA.
But if you want 1920x1200 with 4x AA, then you'll need to move up to a GTX260 or 4870 1gb.(and you still cant max Crysis in DX10 / 1920x1200 / 4xAA / 16x aniso)
My advice would be to wait until you have your monitor hooked up to your 512mb 8800GTS and it may surprise you!
At worst, if youre the type that is really picky about framerates , you'll have to back down to 1680x1050 in some games.
Dont let the people that tell you it will look like crap if you're not running in your monitors native rez of 1920x1200.
Most LCDs scale very nicely nowadays and its VERY hard to notice any image quality drop when not running in native rez.
Now im not biased either way to AMD/Ati or Nvidia, i buy whoever has the fastest card when im ready to upgrade.
Doesnt matter to me who makes it, its how well my games play that matters!!
Ive had crossfire setups and sli setups, depending what was the better buy at the time.
Bottom line is
right now the GTX260 core 16 performs better than the 4870 1gb
The core 216 is faster and Nvidia has been quicker with updated drivers for the latest games than AMD/ATI.(
lately anyway but obviously nobody has a crystal ball for the future)
So you do this....
1: Does AA at a high rez really matter to you?
2: does the price difference in the cards you are considering matter to you? And how much?
Because both the 48701gb and GTX260 216 will max any current game easily.
(with the exception of Crysis again of course)
3: Do you buy games as soon as they are released and expect to play them at max framerates right away, or do you not mind waiting for a "hotfix" and having to "tweak" a bit until a new driver is available. (dont laugh, a lot of us do more tweaking than gaming...its fun for some of us
)
Seriously, you have a great card for gaming in current games now.
And either of the two cards you asked about will allow you to max any current game and by the time one of them cant "cut it" any more, most likely the other card wont either.
So go by ..
price
performance
drivers
Write those down and list them from 1st to last in order of importance to
YOU and you'll know which card is "right for you".