Not sure if there is a specific version number, most of the roms that I have tried from various places have worked fine though. Make sure that you include the neogeo (neogeo.zip) bios in your roms folder though.

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This ROM features various systems, video, and audio tests for the Neo-Geo system. • neo-geo.rom > vs-bios.rom • ng-lo.rom > 000-lo.lo • ng-sfix.rom > sfix.sxf • ng-sm1.rom > delete, not needed Step 5 Copy the renamed BIOS files to your SD card, in NeoGeo > roms. The roms folder was already part of the GNGeo2x zip, so it should have been there along with a readme file telling you to put the BIOS files there.

A full working bios should include the following files: 000-lo.lo asia-s3.rom filelist.txt japan-j3.bin neo-geo.rom ng-lo.rom ng-sfix.rom ng-sm1.rom sfix.sfix sm1.sm1 sp-1v1_3db8c.bin sp-45.sp1 sp-e.sp1 sp-j2.sp1 sp-s.sp1 sp-s2.sp1 sp-u2.sp1 sp1.jipan.1024 uni-bios_1_0.rom uni-bios_1_1.rom uni-bios_1_2.rom uni-bios_1_2o.rom uni-bios_1_3.rom uni-bios_2_0.rom uni-bios_2_1.rom uni-bios_2_2.rom uni-bios_2_3.rom uni-bios_2_3o.rom uni-bios_3_0.rom uni-bios_3_1.rom vs-bios.rom •.