I go with the up-front brigade. Lying is a short-term solution to a permanent problem. The reason she notices it because you have a history of hiding purchases. Just get it over with, pull the band-aid off this one, scab and all.
For the future, I would suggest complain about the old computer vocally, talk up the benefits of the new computer eventually she will get the hint and ask about replacement. Tell her the cost of replacement of the old computer and then mention the extra cost of the better, safer.... dream computer. It is much easier to spend an extra $150 for an upgrade rather than $500 for a new computer. When she asks you about the cost she will already accept there is a going to an expense (with the added benefit of you not whining anymore). Once she is resigned to the fact that $350 is needed, bridging the gap gets a lot easier.
For the current computer just tell her it was an impulse buy because of the up coming vacation and that the old computer was getting wanky. You weren't sure it was going to last and didn't want to get hit paying retail in a tourist trap to keep diving (besides, you got a great discount).