Hey,
This is a tough question because the old iMac has some advantages over the new one, as much as the new one has some advantages on the old one.
Here's why...
The new iMac has a better Video Card and the screen is brighter than the previous iMac.
The new iMac has built in iSight ( but you already have an iSight camera so it doesn't really matter)
The new iMac does not come with built in dial up modem (most people nowadays do not use it, however, if you are a dial up user u will have to pay $49.00 for an external usb dial up modem)
The old 2.0 Ghz iMac has more frontbus than the new 17inch iMac, so it will be a little slower but the new iMac comes with DDR2 Ram which is faster than the previous DDR, so it compensates a little more.
The new iMac is thinner than the previous model.
The new iMac comes with Frontrow so if you like to listen to your songs, videos and photos from accross the room and have the benefit of a wireless remote control, you will love it.
Now the new 20 inch 2.1 Ghz iMac is faster than the previous 20 inch iMac, since it has a faster clock, more frontbus and brighter and faster graphics...
I personally do not like buying refurbished items, they never work right. I have never had luck with refurbished stuff. You are pretty much buying a defective unit, that was fixed to be resold, now, have you ever had your computer repaired? They fix it but then something else breaks, because they didn't put the parts back in place the way they should etc etc...so it is the same thing, the chances of a refurbished item malfunctioning is higher than a brand-new item.
Hope this helped....