While this is an incredibly thoughtful gesture on your part you may be setting yourself up for a world of hurt and disappointment. Like other posters a machine is very personal. I would be very angry with my husband if he did this as I am currently extremely pleased with the machine I have now, as your wife may be with hers. She may secretly covet something else and not want the hassle associated with a new machine. Please seek out this posting:
http://www.quiltingboard.com/t-28927-1.htm for some insite about this kind of "surprise".