The are the same gpu at the core (computer chip), there are three differences.

1) The ATI Mobility hd5730 is running at 650 mhz for the core while the desktop ATI HD5670 runs at 775 mhz for the core (a difference of 19%)
2) The ATI Mobility 5730 is using ddr3 and gets 25.6 GBPS bandwidth vs the ATI HD5670 which runs gddr5 and gets 64 GBPS
3) The mobility doesn't use the standard form factor that a desktop has it instead uses a form factor called MXM (Mobile PCI Express Module). The gpu that is used in the mobility video cards are bin sorted and are then attached to a different board type that is compatible with MXM. This allows the video card of a laptop to have lower thermals than a desktop card, for example a mobility hd5730 has a tdp of 26w vs a desktop hd5570 have a tdp of 39 and a desktop hd5670 have a tdp of 64.

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That said the closest comparison of an ati mobility hd5730 is an ati desktop hd5570. The have the same calculation units, the same core clock, only difference is the memory clock of an ati hd5570 is 900 mhz (on the ddr3 version) instead of 800mhz on the mobility hd5730.

If you look up at review websites that test games, the ati hd5570 performs about 70 to 75% of the performance of the ati hd5670. The ati hd5670 performs about the same performance as an 9800gt getting about 90 to 105% in games.