Here is my config from virtualbox and ROS 3.25:
Name: Mikrotik Test
Guest OS: Other/Unknown
Config file: /home/testuser/.VirtualBox/Machines/Mikrotik Test/Mikrotik Test.xml
Memory size: 64MB
VRAM size: 5MB
Boot menu mode: message and menu
ACPI: off
IOAPIC: off
PAE: off
Time offset: 0 ms
Hardw. virt.ext: on
Nested Paging: off
VT-x VPID: off
State: powered off (since 2009-06-30T07:29:22.149000000)
Monitor count: 1
3D Acceleration: off
Floppy: empty
SATA: disabled
IDE Controller: PIIX4
Primary master: /home/testuser/.VirtualBox/HardDisks/Mikrotik Test.vdi (UUID: a6db4740-da9d-4c00-bce8-4305a3b00cf6)
DVD: empty
NIC 1: MAC: 0800274EE06A, Attachment: Bridged Interface 'eth0', Cable connected: on, Trace: off (file: none), Type: Am79C973, Reported speed: 0 Mbps
NIC 2: disabled
NIC 3: disabled
NIC 4: disabled
NIC 5: disabled
NIC 6: disabled
NIC 7: disabled
NIC 8: disabled
UART 1: disabled
UART 2: disabled
Audio: disabled
Clipboard Mode: Bidirectional
VRDP: disabled
USB: disabled
USB Device Filters: <none>
Shared folders: <none>
Guest:
Statistics update: disabled
I'm using VBox 2.2.4 r47978 and Kubuntu 8.04.1 on a Dell D531 laptop. It's an AMD Turion64 X2. Virtualization is Enabled in the BIOS. Installed ROS 3.25 fresh from the ISO. As mentioned, I couldn't get the Intel virt NICs working, (the nic would show up but no DHCP responses or anything), PCNet seemed to work fine. I installed all packages in ROS except (routerboard,*-test,xen).
I had all problems mentioned above, but turning ON virt. in the BIOS seems to have fixed all problems.
Hope this helps,