If I was you I'd of gone for the stock i5 model and get yourself a 970 from a 3rd party. Dell charge over the odds for their graphic cards and it's debatable whether any games actually make use of the extra cores in the i7. Google i5 vs i7 for gaming.
Regarding the 760ti it should be good enough, I have currently have a Zotac 760 4GB reference seems to play most games at 1080p 50-60fps, only gripe is that it's abit too noisy. Looking to upgrade to 970 or 980 at some point though.
Having said that since i upgraded the bios from A06 to A07 I've started experiencing intermittent excessive fan noise from the X51's front internal fan, it's really annoying as it's very loud, way more than the 760. Should of stayed with A06, however I guess Dell are attempting to improve temperatures within inside the case as with A06 the case did creak abit when it got hot, however now doesn't appear todo this with A07. Note my X51 is the R2.
