Beyond160 has a free trial. I suggest you try it. Since I haven't used it, I can't help you with installation or use questions.
Reading the reviews I saw more negatives then what I could handle. I'll just live with the 160 characters, and use MMS when need be, or wait until Nov when I can upgrade my BB to whatever is the latest Curve then. I sure hope the latest curve will bring back the flash.
There is no guarantee that a later model BlackBerry will support texting beyond 160 chars unless you are willing to change carriers to one that does. The capability in BlackBerry devices seems to be more common with GSM carriers. Or maybe it just seems that way to me since I'm with a CDMA carrier.
Posted from my CrackBerry at wapforums.crackberry.com