I just recently stepped up from a 65nm GTX 260 (192sp) to a 55nm GTX 260 (216sp) for like $17 (was like $9.50 to ship it to them, then $7.50 for them to ship to me, my card actually retailed for more). I felt like that was a good deal.
Like the above poster asked, how much will the step-up cost? Hard to answer your question when we don't know the negatives of your consideration.