I'd suggest not even telling her, and when she notices, just be like: "Oh my good golly gosh, now how did THAT get here!?" Blame the computer fairy.
You could also pretend that you just got a new TFT and a new CPU cabinet. She would not even know whats inside the CPU.
You could always pull off "remember last week when I was telling you about how multi-processor cores would eventually lead to more complex information in-flow devices. It was right around the time that I had a revelation about the pagerank algorithm that I have been missing all this time - you were just about to get on the phone with Jessica when I told you and you were like "that's cool, just make sure you get a cordless mouse" which I thought was totally wierd, but look - see I got the cordless mouse just for you." But you know - stretch it out a little longer with nerd terms that will make her want the conversation to be over.
why?! will she get mad if she founds out that you bought a new computer? tell her today... and mention why did you bought it she'll understand...