The SMS and VMs both show in the same inbox in Google Voice. I use it to send SMS all the time.
To send an SMS to an individual, Open Google Voice --> Menu Key --> Compose --> start typing person's name --> - you're ow composing an SMS.
Of course, I think there are some related setting you'll want to check out in your Voice account first before you start. In particular, if you want to not have to pay for SMS, then you'll want to disable SMS forwarding to your cell phone, and you need your cell phone number tied to your GV account.
HTH