2gb is not really enough to run os x and a VM. it'll run, but as you noticed it will be slow. you should upgrade, but check what slots are free. more than likely you'll have to buy 2 2GB sticks since there are probably 1GB sticks in each of the two slots, meaning there won't be any room for new ones.
i have a total of 4gb on my late '08 MBP (2.4 GHz C2D). i assign 1gb to the VM and things run fine. on occasion a CPU-chewing operation on the VM or mac will slow things down, but that's not too often.
IO (hard drive) is another common bottleneck for computers. what type of drive do you have? the original one that came with the mac? you might consider upgrading that too and/or moving your VM to an external drive if you can. i upgraded to a samsung SSD and notice a world of difference when i occasionally switch back to the platter-based hdd.
but again, upgrading the amount of RAM you have will be the quickest and easiest way to see some improvement.
Last edited: Nov 20, 2012