Have you been fully direct with her? It would be the easiest course of action. First off, you present why you want this thing. Your friends have it, you won't need to go out with your friends, you'd stay home more often, you're tired after work and you want to relax, etc. You need to push these points before you actually argue, otherwise you might not get them in before the actual argument. The argument generally runs as follows...
<EDIT> PARAPHRASE, OBVIOUSLY </EDIT>
"I really want this thing. What's wrong with it? How is it different from the $300 video card you allowed me to buy before?"
--> She gives a dismissing, non-reply that doesn't address the topic at hand
"I really don't want this thing to be an object of tension between us, and it would be a really silly thing to argue over...so I really want to know your side of the argument before I go out and buy this thing. I'm really willing to listen to it." <<This is where the line by line generally degenerates into an OMG-WE-HAVEN'T BEEN-TALKING-FOR-2-WEEKS argument, but if you're lucky...>>
--> Another failure to reply
"Well, uhm. Okay. So, I'm guessing you're okay with it?"
--> (Usually thi "No. You're not having an XBOX."
"Why not?"
--> *shrugs*
-- If the conversation does reach this point, you might have a problem --
"So...you're choosing not to back up your side of the argument?" (Basically, the mentality behind this point is that by not backing up her points with evidence, she drops those points, and therefore your resolution still stands. This is basic speech/debate tactic).
--> Her mentality at this point is that you ought to value what she says over what you want. If she doesn't confront you, dismisses you again, or anything of that sort, then you are very correct in saying the following (Good god as nicely as possible)...
"Well if you're not going to give me constructive criticism I'ma go out and buy it."
At this point she might actually make a rebuttal. But if not, and she resents you for this, you can use this as fuel for a LACK-OF-COMMUNICATION argument (which you will then have in approximately 2 weeks).
EDIT:
Mind you, this Xbox dialog can be transferred to other arguments.
EDITEDIT:
My girlfriend is hating on me for typing this.
EDITEDITEDIT:
But if she really does try to make a case against your argument, do listen to it. Don't act in the same manner as your wife does.
EDITEDITEDITEDIT:
And don't reward her with the idea that she has won through this passive argumentation. If anything, if she makes a realyl good and valid point, you ought to reward her by NOT getting the xbox at all, and doing as she says. That way, you condition her to make more sound arguments, which bolsters communication in the future.
EDITX5:
My girlfriend pats me on the back.