If your budget is limited to $150, and your HD 4890 is dying, yeah, get the GTX 650 Ti. However, don't expect it to be a serious upgrade from what you have.
For example the HD 4890 is in fact better in some games. E.g. in Crysis Warhead at 1920x1200, the HD 4890 gets 31 fps and the GTX 650 gets only 25 fps. http://www.anandtech.com/show/2745/17http://www.anandtech.com/show/6359/the-nvidia-geforce-gtx-650-ti-review/7
If your HD 4890 is still working, and you want an upgrade rather than a replacement, I'd say wait a bit. Save more money, prices will drop a bit too, then upgrade to GTX 660 Ti, Radeon 7870, GTX 670 or Radeon 7950. http://www.anandtech.com/show/6159/the-geforce-gtx-660-ti-review/7