I have been using home studio 9 with an older desktop with a Turtle Beach Santa cruz sound card and a keyboard controller plugged into the Santa Cruz card's midi port. I'd like to upgrade both software and hardware. Updating to Sonar appears to require more computer than I have. What would I need in order to use Sonar with my new laptop? It has no midi sound card, no MIDI inputs, only USB and audio out. Is there a hardware box I can plug into the USB port that would have a MIDI synth that I could trigger with my keyboard?

Welcome, John.
To answer your question, for the most part, yest there is.
 
Is there a hardware box I can plug into the USB port ....
Yes. There are dozens of them available. You are looking for a USB Audio/MIDI Interface.
These come in several configurations. If your controller has only MIDI(DIN) out, you will need an interface with a standard MIDI input. Pricing will vary depending on number and type of inputs, and overall quality.
Check here to get an idea of what's available:
http://www.sweetwater.com/c695--USB_Audio_Interfaces/pn2
 
...that would have a MIDI synth that I could trigger with my keyboard?
No. But that's actually not a problem. The Santa Cruz was a Soundblaster clone, and as such, had an onboard synth.
None of the current interfaces have built-in synths, but they are not needed.
VST synths work within the Sonar framework, and offer much more flexibility.
All versions of Sonar X3(the current version) include some VST synths. You can also use  VSTs from other vendors.
This give you huge options, and just about any sound you can imagine is at your fingertips.

If you purchase a Calkwalk/Roland/ usb audio interface it will come with Sonar (x1) LE. 
This will keep you on familiar ground. Most audio interfaces will come with a basic version of DAW software. Currently Tascam interfaces still come with CubaseLE as an example, but I would expect this will soon change as Sonar and Tascam are in partnership now. 
I just bought a Roland Keyboard ( A49) and it came with Sonar x1 LE as an example. This is good value added bonus I think. It will allow me to purchase the full versions for a substantial discount. 
 
As Robert is saying, you don't need to use outboard or sound card midi sounds if you have soft synths, they are by far superior and unlimited sounds available now, most for free.