I think you left to much out for anyone to provide any real recommendation. For instance, what type of budget are you on? And, I am assuming that there are a couple of Dell models you're leaning towards.
From what I know of America's Army, it isn't that graphically hard on a PC - but in the future you may consider another game that's harder on a PC than America's Army. I always recommend to plan for the future. Give us some additional information cdntrooper.
Ok, with that kind of a budget, you would real nice to build an i7 system.
This build comes to $2493. It doesn't include a monitor. If you need one, you can modify something or look for a different combo deal. You won't find the parts like this in a dell system.
It's really not as hard as it may seem. The build I listed above is all compatible, so you don't have to worry about that. It's pretty easy to hook everything up. Then all you have to do is install windows and the necessary drivers. That's it, you're done.
If you run into problems as some first time builders do, we are all here to help. There are many very knowledgeable posters on here who will help you all the way through. It's much more satisfying to know you built it yourself and didn't get ripped off by a retailer. You will also end up with a much better build for your money.
If you don't want to build one yourself, I can understand that too.
It's hard to say one or the other is a better video card b/c there are several different video cards by each manufacturer. If I had to pick one, I would say NVidia is better because their single gtx280 is comparable to the 4870 x2 ati cards. However, NVidia cards also cost more. I usually go for the 1Gb cards, but it's all about how much you want to spend, and what you want them to be able to do.
The NIC card is unnecessary as the mobo will have ethernet capabilities.
physx will help some games run better, but it's not necessary.
If you want a pc built for you, instead of going with dell, I would recommend this company.
They can build a higher end pc and you can choose what you want included. There are some i7 builds for under $1500.
For the power supply, you would want to look for something ~400w. If you have any thoughts of adding a 2nd video card, then you would want a psu ~750w. The good brands are antec, PC power & cooling, OCZ, and seasonic.
Good mobo manufacturers are Gigabyte, Asus, and EVGA. If you are getting a quad or dual core, then the p45 chipset is a good choice. If you go with the i7, then look for the x58.