is that model Dell overclock-able.?
a few XPS models Dell allowed them to be overclocked..
GTX 460 would be the most I would run on your current chip, it's a nice pairing.
Instead of spending a lot of money on the GTX 560 I'd look into upgrading your current processor instead.
A Q9500 and up can cost almost $200.
C2D E6850 @ 3.0GHz is a solid mid-level chip, but you'll need more.
C2D E8400 and up is also an upgrade from your current chip.
not saying that it's worth it to you or some, just letting you know.
Unfortunately it is not overclockable. I also will not be upgrading the cpu, so I guess the best choice would be the GTX 460.
Thanks so much for the quick responses.