The worst parts of say, the 2010 iMac right now are:
1. Hard Drive
2. Out dated GFX
So, I would suggest at the minimum, getting an SSD in your new iMac, and getting the upgraded GFX card.
The quad core i5 in the 2010 iMacs is still blazing fast compared to other stuff even out now. Chances are in 5 years from now, 700mbs on an SSD wont be too bad, your quad core i5 will still be fast enough, but your graphics card will be the bottleneck. So, to dampen the hit on the gfx card in the future, I would definitely buy the higher end card now.
1. Hard Drive
2. Out dated GFX
So, I would suggest at the minimum, getting an SSD in your new iMac, and getting the upgraded GFX card.
The quad core i5 in the 2010 iMacs is still blazing fast compared to other stuff even out now. Chances are in 5 years from now, 700mbs on an SSD wont be too bad, your quad core i5 will still be fast enough, but your graphics card will be the bottleneck. So, to dampen the hit on the gfx card in the future, I would definitely buy the higher end card now.