As others have said, if the system is enough for your needs (and judging by those specs it surely will be), then go ahead and enjoy your iMac.
Sure, the new model could be around the corner going by the Google ad's being leaked, but unless you're interested in a slightly thinner iMac and possibly a Blu-Ray drive if you believe the more extreme rumours, but I don't see the iMac being revolutionised.
I got my new iMac in March this year for a bargain £612 through a work rebate (it was the 20" that RRP'd for £782 at the time), so it was a great saving. However, at the time I got mine the update went ahead and the only cosmetic change was the thinner "foot" ... not exactly a deal breaker.
The specs were increased a little, not much on the CPU speed but the GPU was changed from an ATI to an nVidia 9400M ... again, I'm not a gamer and for my Photoshop work the ATI Radion 2400 HD handles everything fine.
I know people are clamouring for Quad Core iMac's, but the funny thing is that the majority of people clamouring for them are only going to surf the net, use iTunes, and play the odd game. I personally think Core 2 Duo's are ample for the casual users who are attracted by the iMac, because real power users are always going to look at the Mac Pro and that's where Quad Core comes into things.
Again, just my opinion, but Apple tend to move quicker than any other manufacturer on the market when it comes to their SKU's, which is why it's easier to just buy a system you need and that will do you a few years.
After that, simply enjoy it.
