As far as I know (which is only a few..lol), mobility GPUs are usually "weaker" than its desktop twins (same first 2 series number)
Since you have a crossfired 2xHD5770 setup, the closest one is the one you don't want to have since it will defeat a purpose of a laptop, the 58xx series.
The best compromise is to have an ATI HD5730m, the reasons are simple: you already predetermined the nominees and qualifications, phasing out the 58xx. you're also familiar with ATI's products on your desktop. So, as a 1st laptop, accustoming would be a stroll in the park. You'll know better than anyone not to expect a laptop to match up your crossfired desktop PC's muscle power. It should run current games pretty well though, especially the ones dubbed "gaming notebooks".
Hope this helps!