If you’ve tried enlarging in Photoshop, the first thing is to experiment with the resampling algorithm (photoshop suggests bicubic smoother as the best for enlarging, but I have found it to be image dependent (if you have an image with a lot of edges vs a portrait or landscape).
Rather than smoothing, blurring I would suggest using a denoise program next, because they are smarter than any layer tricks, and can denoise while retaining sharpness.
Or you can use a product that is made specifically for this purpose, like Genuine Fractals. It promises up to 1000% enlargement without loss of quality.
Jeff Atwood has a blog post about this here: Better Image Resizing