Hi

I have the following setup

ether1 - connection to wan (bridged modem using pppoe dialup on mikrotik router)
ether2 - 192.168.2.0
ether3 - 192.168.3.0

I would like to have my internet traffic prioritized in such a way that the subnet 192.168.3.0 has full access to my 4 meg/ 512k internet connection.
However, when any computer from subnet 192.168.2.0 wants traffic it should get full priority over the same bandwidth and computers on the 192.168.3.0 subnet should have traffic queued.

Effectively if 192.168.2.0 wants to use the entire 4 meg bandwidth it should be able to do so. Or as close as possible to this.

Have a look at the information on queuing:

http://wiki.mikrotik.com/wiki/Manual:Queue

I have looked at that and spent hours trying to make it work. I failed. How do I specify subnets over single ip?

Also when I tried to do this with two IP addresses with different priorities it ended up splitting the traffic equally

If your subnets are on different interfaces why not apply the queues on the interfaces instead the subnets ?