Never said a 7750/7850 was top of the line, that would be pretty foolish. But a bottleneck is a bottleneck. Any really nice video card will still be bottlenecked by the Celeron, not so much for Crysis requirements.
Besides, at 1024x768, a GTX 260 would be overkill if you ask me, the GTS 250 is more than enough to handle it, especially for that cost.