Yes and no. The only phones that allow you to set a ringtone directly from you computer on to your memory card as a ringtone are PDA devices. More accurately the Blackberry. However if you have a data plan you can send the MP3's through a site called Funformobile.com and they will convert it into a ringtone and even send it to your phone for you, all for free. Just keep in mind you do need a data plan set up with Sprint to do this.
I've also been using Myxer.com with great success. However, do NOT set the ringtones if you send them from Myxer to anything other than the default phone setting or they distort BADLY. I'm hoping I can figure out a way around that but it's not looking good.