First off, you should realize basically noone on here is a "professional". So if you're determined to get such advice, you should go elsewhere. However, from what I've seen, most of us know more than "professionals" elsewhere...at least at places like Best Buy and such!
So basically out of this whole post, you want a computer that can play games decently, right? Nearly all of this is based upon your graphics card. Unless you get one of Dell's high end system's, their graphics cards are very low end, and probably won't be any better than your current machine.
My suggestion would be to either build a computer, or get a Dell and purchase a graphics card from somewhere like Newegg.com. The card I have in my computer was only $160 and it's MUCH more powerful than a 7300(which is probably the best of the few you posted)
Now, if you must get everything from Dell for some reason, your best bet would be the 7300GT. However, it's still a very low end card.
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Desktop * Athlon X2 4200 | 2G RAM | 160G HD | 7600GT
Laptop1 * C2D T5550 | 3G RAM | 120G HD | Intel x3100
Laptop2 * C2D T5250 | 2G RAM | 160G HD | Intel x3100
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