2Andi B.:
v.5.0.51 is an unofficial leaked version (though, it works quite well). I uploaded it for someone for testing purposes. So, it is not listed on wiki page and it does not include README. README is a bit updated, but not significantly, so a README from latest official version can be used. The current README is always located in the SVN repository in sources:
http://trac.netlabs.org/vbox/browser/trunk/doc/ReadMe-OS2.txt (note that SVN trunk is a bit newer version, not yet working and not yet released).
VT-x/AMD-V can be enabled in newly created VM if your system supports hardware virtualization. Though, if it is not the OS/4 kernel, hardware virtualization will not work and the VM will likely panic, if you enabled it. So, if you don't have the OS/4 kernel or your system does not sipport VT-x/AMD-V (like my main system, I have Athlon 64, single core Socket 939 without AMD-V), you should disable it too. You simply need to uncheck the checkbox AMD-V/VT-x. PAE/NX can be checked, it should always work, and doesn't caause VM to crash. IO-APIC is recommended to be disabled for WinXP guest. Otherwise, it may be useful in some cases (depends on a guest OS). NIC depends on guest OS too (for OS/2 guest this may be Intel 1000/Pro MT, as well as AMD PCNet). The only network sharing mode working is simple NAT. TAP driver is currently broken. OS/4 kernel is needed for hardware virtualization support. Note that the official version avaiable for download from the wiki page, does not include hardware virtualization. Unofficial v.5.0.51 supports it. Nothing special is needed. Just enable AMD-V/VT-x in BIOS and in VBox settings. Otherwise, these two versions should be the same.
OK, I'll try to update the wiki. I thought it is evident thing that hardware virtualization is unsupported on IBM's kernels. As it never worked on them and it lacks the "CPU rendez-vous" feature which alows to run code on all CPU cores in parallel. This is essential for enabling AMD-V/VT-x. So, it is experimental feature with OS/4 kernels. Of course, VBox enables it if it finds that it is enabled in BIOS and it is supported by your prosessor. But it cannot check which kernel you have installed.