First off
OMG! you paid 3 bucks for a ring tone?!?!?!?!?!?!? Please next time if you don't have the song and want it as a ring tone JUST ASK in the ring tone request thread

If you already have it (as an uprotected song) read the tutorials on how to make ring tones...
Try ToneFX, you can get it Cydia, it's simple to use and even has it's own importer to put on your PC to load the files into the app and create them too. The ONLY draw back is it costs a donation $5.00 min. thru paypal. But they do offer a trail period so you can get it and see if you like it.. Heres their site
ToneFXs
Otherwise you need to make a under 12 sec. sound file save/convert it to a .caf name it the same as the one you want to replace. To do this you need to SSH your phone open the file: Root (the main root file)/System/Library/Audio/UISounds
Now by text do you mean the text message sounds ?
Those would be the
sms-received6.caf
sms-received1.caf
sms-received2.caf
sms-received3.caf
sms-received4.caf
sms-received5.caf
Files you need to replace
If you are talking about the the typing sounds it's the
Tock.caf
file you need to replace
I would suggest to just get the ToneFXs app and get their program for your PC to load the tones. It is much easier (and cheaper) and you can change the tones at any time.
If you do SSH ALWAYS MAKE A BACKUP COPY OF THE FILES YOU REPLACE!!!