my laptop is a 2009 mid 13inch mbp 2.26 c2duo, upgraded to 8gb ram , 2x 500gb hds, slow as ****, but took me thru med school and residency, hardly use it nowadays. Now my plex server and photo repository can be moved to the Imac, and ill get a new 15 inch later.
Pop an SSD in that puppy and I think you'll find that it ain't slow as **** anymore. The 2009 is very easy to upgrade in this fashion too. I have a 2009 Unibody, a 2012 Unibody, and a 2016 tbMBP. I'm not doing any serious data crunching on the 2009, but for regular tasks, I'm not gritting my teeth waiting for the beachball to go away. It's pretty snappy ever since I swapped in an SSD.
As for the iMac, the wisdom around here always goes "buy it if you need it and stop second guessing your decision, but if you can wait and want to wait, hold off till the next release."
At this point of hardware evolution, we're not seeing paradigm shifting updates anymore. Everything is incremental at this point, but as someone else said, if they're offering bigger SSD drives, that's a pretty big deal.
If you can get by on your old machine with some speed improvements a la SSD drive, then maybe you should return it and wait. You can get a new HP 120GB SSD for $25 on eBay! A plug and play "toaster" oven interface for those 500GB hard drives you have would then run you around $30 or so. So if your needs would be met this way, go ahead and return it and wait out the new ones whenever that might be. If they underwhelm, the last gen is likely to be available at a discount or refurb units could be had so you may not be losing out on pricing.
On the other hand, if that new iMac serves a real need today, just keep it.