I don't really see any reason to upgrade, personally. I can't see that machine having troubles with any game on the market really. If you feel the need to upgrade, then it'd be up to you to decide whether or not it's worth it to you.
I too vote in favour of 'no upgrade needed' on the listed items atleast for another 6 months to 1 year.. Which are your other system components.? May be you can upgrade to a better case and/or a better PSU etc..etc..etc..
Stupid Dell with their locked down bioses... Anyways, I think many nForced based motherboards can be software overclocked... if your lucky your Q9540 might get a good overclock out of it. The XPS 650 isn't showing up on the US Dell site..
I wouldn't bother upgrading the CPU. I really doubt you'd be able to notice any difference. Upgrading the GPU might make some sense. What resolution do you game at? Do you want to get into multi display gaming? Without knowing your monitor resolution its hard to say if you need a GPU upgrade. The 4970 is/was a great card so it should be handling everything ok.
BTW, rumor says the 6870 will be slower then the 5870. The 6870 is supposed to be just slightly faster then the 5850.