With standard Photoshop techniques (USM & Bicubic interpolation), I have to work hard (extra layers with gaussian blurring to smooth out "wigglies" from upsizing & to soften USM artifacts, etc.) to get good 11x14 from my 3.2 megapixel camera. But using the following techniques give a wonderful 18x24 with no extra work:

Use convolution sharpening instead of USM (see
http://www.aim-dtp.net/aim/techniques/index.htm
under "Better Than USM"). Amazing to see so much more detail in shadows. And what completely won me over was seeing teeth in a group portrait - enough USM for the rest of the picture ruined the teeth (similar to severe JPEG blocking artifacts), whereas convolution sharpening gave a very sharp but smooth natural look.

For upsizing, use the Jensen-Xin Li Hybrid of SAR (see
http://www.general-cathexis.com/ ).
It does a great job of edge enhancement while also smoothing out "wigglies" other upsizing algorithms produce. I should get the documentation for it to figure out all the parameters, but the defaults work pretty darn good! Wonderfully sharp, smooth edges.

Occasionally, it gets misled into thinking there's an edge when it's really just a contour transition and will "enhance" the edge. So I generate a Lanczos enlargement also (from BreezeBrowser - you could use a bicubic interpolation from Photoshop instead, though it's not quite as good), put it as a layer under the SAR enlargement, and using a layer mask, expose the Lanczos version where the SAR algorithm got too heavy handed.

Strangely enough, sharpen BEFORE doing the upsizing when using SAR. The sharper edges actually help the algorithm determine and enhance edges.

May I also suggest "Clarifying" your image before anything else. This is Unsharp Mask with Amount: 15, Radius: 30 (maybe 50 for your 5 mp), and Threshold: 0. This will add some local contrast to medium and small detail without changing the global contrast of the picture. It's very similar to what film does, enhancing the edge effects. Gives a more 3 dimensional look to the image (and isn't that the biggest complaint film aficionados have against digital - that flat 2 dimensional look?). Search for "Clarify" and you'll see numerous posts about it, some with examples. Try it, you'll like it.

Shoot RAW at lowest ISO. Starting with low compression, low ISO JPEG is ok for 8x10 (maybe 11x14 with your 5 MP camera), but any larger and you're just making your job a LOT harder. Those JPEG artifacts that were supposed to be invisible suddenly become quite visible when enlarged that much.

When you're done, feel free to save the final result as a high quality JPEG. Since you've already blown up & sharpened the picture, what the JPEG algorithm determines to be too small to notice will remain too small to notice (as long as you don't go using curves, upsizing, etc. in the future on it to bring those artifacts back out).

Using these techniques, I've printed wonderful 16x20's and 18x24's from my 3.2 megapixel Canon S30 - enough better than USM & bicubic upsizing that I'm redoing everything I had already done larger than 8x10. So you should be able to make great 16x20's from your 5 mp camera.