Okay I'll have a stab at this:
1st: Decide how important the extra screen real estate a 20" will provide is to you. (For me, I really appreciate that screen in my work)
2nd: Decide how you are going to use the computer. If its going in your bedroom/office etc. and you aim to primarily use it for your own home/office tasks then after the initial flush of excitement you'll likely hardly ever fire up FrontRow. If you would really like it as the main media hub (e.g. you live in a dorm etc.) then sure, the new features will be handy.
3rd: Think about your use of iSight. Are you going to get major use out of it? Do you have people to video chat to in iChat such that you can really enjoy it? IF so, would being able to indepentantly position the iSight be important to you or would you only ever put in on top of the iMac in one position?
4th: Would you likely purchase a separate external HardDrive for backup security (say, a LaCie) or are you fine loading everything on the internal?
Frankly I'd personally discount the marginal speed increase and new GPU from your decision. In my opinion they are not compelling enough factors at this stage. The new machines are really, really cool, but you have to try and divorce yourself from the Apple/Jobs reality distortion field and really think how you are going to use the machine. Only then can you decide if the new features will really enhance your use of the machine or be so much eye candy.
cheers
Vanilla