It all comes down to the Graphics cards.... which one?

Ok. so at this point I'm stumped as to which computer i want to get. the fight is between a Dell XPS 1530 and an HP HDX16t.

The specs are almost exactly the same duel 2.5ghz processors, 4mb ram, 7200Rpm processors, but the difference is the graphics cards

the XPs 1530 has a 256mb GeForce 8600
the HP has a 512mb GeForce 9600

I've read from another post that the thousand digit (dell=8, hp=9) is a model number and doesn't matter, making the cards preformance exactly the same. i read this on a post comparing a 512mb 9200 to a 256mb 8600, but my question is now, considering that a 9600 is similar preformance wise to the 8600, how much better is the 256 more mbs?

Also note: I'm planning on keeping this computer for a LONG time (8 years maybe, which is the life of the one i have now). If that matters for the span i can do good gaming. Also, I'm bias for dell. my family has been a dell family for 8 years, which is the length of time I've had my dell.

If you could also list some general advantages or disadvantages for each model (dell xps 1530 vs. hp Dhx16t) that would be really awesome

If you only have those two choices, the HP with the GeForce 9600 is what I'd recommend.