I am not sure what you budget allows, but form what I have read, the 24" iMac not only has a bigger display, but it's of much higher quality then the 20". I know it's a deference of $500 or so dollars, but it might make up the deference in the long run. Now if you are coming form an eMac, the 20" will probably look gorgeous and be quite capable for your needs. Ether way, you going to love Leopard!

Now as far as choosing from a MacBook to an iMac, just remember, if you do go for the laptop, they usually are a bit more prone to "accidents" then the desktop. I would strongly suggest you get Apple care if you go for the MacBook. Also, they are more susceptible to get stolen. If the work you do does not require you to be mobile, I think you will be better served by the iMac. On the other hand, if you like to get out of the house and work anywhere, the laptop is more for you. As far as capabilities, they are pretty similar as far as feature sets and speed.

Get less RAM and upgrade as you budget allows. 2 Gigs will get you going from the start, you can later swap the RAM to get the 4 Gigs. The upgrade is not that costly and you will have more time to save money for it. The CPU however is not upgradable and you are stuck with what you get.