It's pretty likely that they won't update the software if they come out with a new one because the hardware will be different.
That said, all we have to go on is rumors right now, and there is no real evidence of it happening soon (despite what Kevin Rose might say). I do think there is credibility that there is a new one in the works, but nobody knows when it will be ready. It could be next month, or it could be next year.
If you are absolutely sure you want the newest one or whatever, and you can live without your current one, then I see no reason not to sell it. It's best to sell as early as possible to make as much money as you can before the prices drop.
If you use your current AppleTV enough that you would miss it, you might as well stick with it until there is a lot more evidence pointing to a release date.
It is really up to you and how you would feel living without your current one.