There are various lenses out there at varying prices for varying photo styles and purposes.
The 55-250 is an entry level zoom lens. Its a good one, don't get me wrong, but its not a professional lens. If you don't have a zoom, this is a good zoom to get. Better yet is the 70-300. Better than that are the 70-200 f/4 and f/2.8 IS. Its all relative.
Personally, I'd go with the 50D over the 500D. The better high ISO, the better construction, LCD, mp,... its just a better camera. It depends on the photography you are doing. For some, the 500D is enough. If you are new, still learning, plan on shooting portraits of friends and the city, then I would think the 500D is enough and I would go with the 55-250 and grip.
By next year, when you might want to upgrade, you can then look at the 50Ds which will be way cheaper.
Instead of the 50D, maybe look into a used 40D. With the new 7D out, i'm seeing more and more 40Ds on the used market.
Keep in mind that in photography, specially when you are starting out, there are so many things out there that are better. And until you can put aside $7500 to buy a camera and lenses, there will be something technically better. If you get the 50D, then there is the 7D...and the 5D...and the 5D Mk II...and now the 1D Mk IV... there is always something more.-