No No..your confused I think..
Nvidia released the 9800gtx (non +)
ATI then released the 4850, which beats the non + 9800gtx.
So nvidia dropped the 9800gtx+, to beat out the 4850
ATI also released the 4870 with the 4850, the 4870 beat out nvidia GTX260.
So nvidia then dropped the GTX260 216 core, which is on par with the 4870.
Now nvidia has already released the newer version of the 9800gtx+, called the GTX250. It's slightly faster then the 9800+, and a touch faster then the 4850.
SO if you got the 9800gtx+, you only step up option would be the GTX250, which is $130. The 9800gtx+ is still going for more then that though, so you'd be loosing money.