My experience trying to share my PCs LAN connection with my N900 over USB was a nightmare but it eventually worked.

Using MAD Developer;
Click on Manage USB
Click on Load G_Ether

Windows (XP) will either create a new LAN new connection using the installed LINUX USB Ethernet Driver
OR
it will detect the new NIC ask you to install the driver , see guides for completing this step.

The above steps are the same as the cmdline steps in #1 and #2 .

I perform step #3 on the N900 but with IP addy 192.168.2.15
I now have
192.168.2.14 -> Windows XP USB NIC
192.168.2.15 -> N900 UCB NIC

From each of these hosts I can ping the other host. You could now establish a VNC connection to your N900 if you have the VNC server installed on your n900 and a VNC client on your PC.

To browse the internet on your N900 you still have some act
ions remaining – primarily sharing your PC LAN/WAN connection – clearly a direct connection to the internet or connected via a router.
I’m using NAT32 Windows Software router to avoid using windows ICS which has a lot of compromises.

Hope that helps.