I bought a pc yesterday with:
8 gb ram
Core i7 2600 4 cores 3.40ghz
Motherboard: OPTIPLEX 390
Power supply 260 watts from intel
Intel hd graphics 2000
And I want a good graphics card that can fit in the computer and run high end games using these watts
Unfortunately with that power supply, you won't be able to get GPUs that are too good, however the one I recommend getting for those specs is the GTX 1030, which should leave you enough power for the rest of your components.
Before going out and spending money on a GT 1030 (it's a GT, not GTX, not that it matters), what top end games are you wanting to play and at what resolution?
Assuming you've got a 1080p monitor @ 60Hz, then the GT 1030 would be fine for lower requirement games. However, if you're wanting to play the latest AAA titles, you need a better graphics card, which also means getting a better power supply.