I used a similar glow technique in my Chrome plugin...

You might want to blur the texture before rendering it back to the screen...

To blur it, you render the texture twice back onto the *same* texture you rendering, first with a shift to the left, then again with a shift to the right
(with transparency of course)

Then you take the updated texture and render it back onto the same texture again, this time shifting it up and down.

If you do this enough times, it will make a VERY nice blurred glow.

Fullsize pic: http://www.nitrogen.za.org/viewimage.asp?id=26