That's exactly how I have my network set up. I have a Linksys router/modem and a 1Tb Time Capsule. I have the TC set to 5Ghz only and have my Linksys set to Wireless G mode. This way, my MBP, MB aTV and iMac all connect through the TC at higher speed for backing up via Time Machine or streaming videos from my iMac iTunes library. My other stuff like iPhone and iPod Touch connect through the Linksys at G speed.
IIRC, you need to launch Airport Utility on your Mac and then select Time Capsule on the left. You will be guided through a series of questions asking if you want to join and existing network or create a new one etc etc. Select create a new one and give it a unique name (I called mine network5G). There should also be an option there to select how you want the TC to broadcast, i.e. Mixed b/g/n or n-only. Selecting n-only will set your TC to only broadcast at 5Ghz.
Hope this helps